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The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers
Accepted Manuscript Title: The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers Authors: Jan Kotˇena, Anton´ın Minaˇr´ık, Erik Wrzecionko, Petr Smolka, Magda Minaˇr´ıkov´a, Martin Minaˇr´ık, Aleˇs Mr´acˇ ek, Ivo Kuˇritka, Michal Machovsk´y PII: DOI: Reference:
S0924-4247(16)30646-X http://dx.doi.org/doi:10.1016/j.sna.2017.05.019 SNA 10128
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
6-10-2016 5-5-2017 9-5-2017
Please cite this article as: Jan Kotˇena, Anton´ın Minaˇr´ık, Erik Wrzecionko, Petr Smolka, Magda Minaˇr´ıkov´a, Martin Minaˇr´ık, Aleˇs Mr´acˇ ek, Ivo Kuˇritka, Michal Machovsk´y, The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers
Jan Kotěna†, Antonín Minařík*†,‡, Erik Wrzecionko†,‡, Petr Smolka†,‡, Magda Minaříková†,‡, Martin Minařík†,‡, Aleš Mráček†,‡, Ivo Kuřitka‡, Michal Machovský‡
†
Department of Physics and Materials Engineering, Tomas Bata University in Zlín, Vavrečkova 275, 760 01 Zlín, Czech Republic
‡
Centre of Polymer Systems, Tomas Bata University in Zlín, Třída Tomáše Bati 5678, 76001 Zlín, Czech Republic
* Corresponding author: E-mail address: [email protected]
Graphical abstract
Highlights
Anisotropic etching of silicon wafer under temperature gradient was studied.
The influence of temperature gradient magnitude and etching time was tested.
Novel etching method with standard etching solution.
High specific surface of the etched wafer resulting in dramatic reflectance decrease.
ABSTRACT A novel approach to wet etching of silicon wafers (100) is described in this study. Homogenous organized surface structures were prepared by the utilization of self-organized flow in the etching solution (Bénard-Marangoni thermocapillar instability). The driving force behind this process is a temperature gradient generated by the etching apparatus exclusively constructed for this research. The influences of temperature gradients (1 to 20 K) and etching time (20 to 55 min) were studied, with the potassium hydroxide / isopropyl alcohol etching solution. Obtained results indicate that self-organized flow can be utilized for specific and rapid etching, which allows significant variation of the topography and reflectivity of the silicon wafer surface textured in this way.
Keywords wet anisotropic etching, silicon, temperature gradients, Bénard-Marangoni thermocapillar instability, reflectivity.
1. Introduction The minimization of energy losses on silicon wafers intended for solar panel assembly has been studied from different perspectives by many researchers [1-11]. One of the critical parameters is the final reflectance of electromagnetic radiation from silicon surfaces modified either by texturing [1, 2, 5, 7, 9, 12-16] or by applying antireflective coating [2, 12]. Surface texturization is usually done by the controlled etching of monocrystalline (Miller indices – (100), (110), (111)) [3, 4, 6, 13, 17, 18] or polycrystalline [7] silicon wafers. For this purpose, acidic [19-21] or alkaline (KOH, NaOH, LiOH) [17] anisotropic solutions are widely used. The rate of this modification depends heavily on temperature [3, 4, 18], concentrations of reactants in the etching solution [3, 4, 6] and the presence of organic or inorganic additives [6, 16]. Addition of alcohol-based substances affects not only the etching rate, but also the homogeneity and distribution of the pyramidal grains [6, 13, 22]. On the other hand, different shapes of surface grains can also be obtained with the assistance of etching masks [9, 11, 14, 23-25]. These different approaches are usually evaluated based on their impact on environment and their costs [26]. Other important factors are the purity of
chemicals, pretreatment of the silicon wafers [26, 27] and circulation of the etching solution [28, 29]. Texturing itself takes place in single-purpose texturing machines, which are able to produce a large numbers of textured surfaces. These machines, however, often contain many moving parts, which is a potential source of contamination. Such problems could be minimized by utilizing forced circulation of the etching medium for effective silicon etching. Hence, the motivation for our investigation was to develop a relatively simple etching method utilizing temperature gradients and thus controlling the circulation of the etching solution. In this paper, we present an alternative etching method that utilizes Bénard-Marangoni convective thermocapillar instability [30, 31] in the etching solution for controllable modification of silicon wafers using temperature gradients. The above mentioned process can effectively and easily etch silicon surface in a similar manner as described in literature [3, 4, 17, 24]. Besides that, by controlling the etching process, various unique structures can be prepared [32]. The silicon wafer surface reflectivity has also been investigated in our study and was correlated with the morphological features of the prepared surfaces. The results have shown a significant improvement in surface reflectance compared with control surfaces prepared without temperature gradients.
2. Experimental section 2.1. Silicon surface modification. Single-crystalline boron-doped p-type silicon wafers (resistivity = 0.01 – 0.02 Ω.cm, radius 150 ± 0.2 mm, thickness 525 ± 15 µm, ON Semiconductor Czech Republic, LTD) were used for the preparation of uniform pyramidal texture. Initially, the wafers were cleaned according to the process described in the literature (RCA clean) [33]. After cleaning, the wafers were washed with pure ethanol and then blown dry with nitrogen. The wafers were then inserted into the etching cell of the apparatus preheated (10 minutes) to the desired temperature (353 K at the wafer/heating plate interface) and etched in a solution containing 2.5 w.% potassium hydroxide (Sigma-Aldrich, KOH basis ≥ 85%), 15 vol.% isopropyl alcohol (Lachema Brno, purity > 99.7 %) and ultra-pure H2O (resistivity > 18 MΩ.cm). Different temperature gradients (measured between wafer surface and a point located 35 mm above it, i. e. above the etching solution level) and etching times were used
in order to influence the final texture. When the etching was finished, the wafers were removed from the etching cell, washed with pure ethanol and blown dry using nitrogen
2.2. The characterization of surface changes. The topography of the etched surfaces was studied with the scanning electron microscope (SEM; -model FEI Nova NanoSEM 450). All SEM images presented in this study were obtained at 5 kV accelerating voltage with the back scattered electrons (BSE) detector in the immersion mode. The purity of presented surfaces was studied using energy dispersive X-Ray analysis (EDXA) method with the above-mentioned SEM. The topographic changes were characterized with the atomic force microscope (AFM; model Dimension ICON from Bruker) as well. All measurements were performed in the tapping mode with the MPP12120 probes, (Bruker). The sampling area was 40x40 μm with 512x512 resolution and 0.5 Hz scan speed. The topographic changes were also studied with the stylus profiler (Bruker – Dektak XT). Surface reflectivity was studied using spectrophotometer (model Perkin Elmer UV/VIS/NIR LAMBDA 1050) with 150 mm InGaAs integrating sphere..
2.3. The etching apparatus. The etching apparatus was assembled for the purpose of this study at the Tomas Bata University in Zlin and utilizes Bénard-Marangoni thermocappilar instability for controllable etching of silicon wafers. It consists of heating and cooling areas with evenly distributed temperatures, inert etching cell with sealing elements and a system for storing condensate, precise instruments for temperature regulation and separately placed ventilation system (Fig. 1). The main advantages are simplicity, the ability to etch with small volumes of etching solution and spontaneous movement of chemical products away from the etched surface. [34] The inner cell of the apparatus has the dimensions 60 x 40 mm (internal diameter x height) and can accommodate a silicon wafer 60 x 60 mm and etched area with diameter 44 mm. The overall dimensions of the apparatus are 150 x 150 x 350 mm (width x depth x height). Typical volume of the etching solution is about 10 ml.
3. Results and discussion The self-organized flow in the etching solution, Bénard-Marangoni thermocapillar instability, is driven through changes in surface tension, which causes temperature fluctuations on a phase boundary between liquid and gas [30, 31]. This flow can be characterized by the equation (1). dT d T Ma κ
(1)
Here, Ma stands for Marangoni number, determining the initiation of the BM-instability, the temperature difference between the bottom layer and an open surface dT, the thickness of the liquid layer d, dynamic viscosity µ and the coefficient of thermal diffusivity κ. The equation (1) shows that not only the absolute temperature, temperature gradient and the thickness of the liquid layer above the etched silicon wafer determine the creation and intensity of the self-organized flow. Our study focuses on the etching time and the temperature gradients. When the sample is subjected to minimal temperature gradient (Fig. 2a), the self-organized flow is not generated. The etching medium and products of the etching reaction fluctuate randomly over the silicon wafer surface, which is caused by Brownian motion and buoyancy forces, when warmer (less dense) liquid rises and colder descends. The Fig. 2b shows the self-organized flow caused by the temperature gradient. In this case, the etching solution is quickly changed over the surface, so no cumulation of reaction products occurs and no further measures their removal are needed as is described in literature [13]. Another increase in the temperature gradient causes secondary flow patterns to emerge among etched pyramidal structures (Fig. 2c). The assumptions for this hypothesis come from our experimental results and from the liquid flow mechanics [35]. The presence of the self-organized flow and its effect on the silicon wafer surface can result in uneven distribution of surface structures (from the perspective of size and type) when temperature gradient exceeds 10 K. As can be seen in the Fig. 3b and 3c, these inhomogeneities are presented on the edges of the etched area. This is caused by uneven distribution of surface tension and various forces near the edge, which was discussed in detail in our previous work focused on polymer systems, where similar inhomogeneity has been observed [36]. Although these inhomogeneities exist, they do not affect the final reflectance in the UV-VIS spectrum nor the etched area coverage, as discussed below. In addition, the etched surfaces were always characterized in the 12 cm2 central area, where these defects are
not present. When small temperature gradients are used (up to 10 K), all surfaces are covered with comparable pyramidal grains from the edge to the central area, as is shown in the circular cut in the Fig. 3a. However, these small temperature gradients do not cause the generation of the secondary flow patterns (Fig. 2c) and the creation of the secondary roughness (Fig. 3b and 3c). The Fig. 4 contains supplemental information to the apparatus, etching process and temperature gradient direction. In this experimental setup, the lower temperature was equal or higher, compared to the upper plate. Temperature gradient was defined as the temperature difference between lower and upper plate. The temperature difference was measured between the wafer/plate interface and a point located 35 mm above it, i. e. above the etching solution level. Also the average temperature of etching solution was monitored (Table 1 and Table 2). With increasing temperature gradient, the average solution temperature decreases (as the substrate temperature stays constant and the temperature of upper plate decreases). Even though the average solution temperature drops from 351 K to 338 K (Table 1 and Table 2), the etching rate increases (etch depth 13.3±0.1m and 23.2±0.2 m, respectively, during 55 min). This suggests dominating role of temperature gradient over the absolute temperature. Volatile components of the etching solution can condensate at the upper (relatively colder) plate, as it is separated from the solution surface with an air gap (the presence of a liquid/gas interface is one of the essential conditions for the occurrence of the Benard-Marangoni convection). This condensate partly reflux to the etching solution, partly stays trapped on the etching cell walls. The control experiments performed with pure KOH solution revealed that structures similar to those in the Fig. 3b etc. cannot be prepared without IPA. This fact suggests, that IPA stays in the etching solution during the whole etching process, though its concentration may decrease. Also, in any experiment the temperature of IPA boiling point (355 K) was not exceeded and the best results were obtained at the temperature 338 K (Fig. 8b). Indeed, the influence of etching solution concentration changes on the “hillocks” formations cannot be neglected [17, 32, 37-39], in our experiments though, it has minor effect compared to the temperature gradient and the organized liquid flow. The reflectance in the UV/VIS spectrum was measured due to the application potential of these surfaces in photovoltaics. For better clearness are the comparisons expressed as a reflectivity difference ΔR between the etched and the reference sample at the wavelength 600 nm (Fig. 6 and 9, Tables 1 and 2). The Fig. 5 demonstrates the effects of the temperature gradients as a driving force behind the variation in surface patterning. The Fig. 5a shows the patterned surface prepared without the
temperature gradient, which creates a surface commonly described in literature [3-5, 13, 17, 24]. Gradual increase in the temperature difference from 5 to 20 K causes changes in the flow mechanism (Fig. 2) and the transition from symmetrical patterns to asymmetrically etched surfaces (Fig. 5). These topographic changes directly influence the measured UV/VIS reflectivity (Fig. 6), which corresponds with the findings presented in the literature [1]. The increase in the temperature gradient value from 1 to 20 K causes more than 5-fold etching rate increase (from 2.3 ± 0.1 to 13.3 ± 0.2 μm) and more than 2.5-fold increase in the surface roughness Sa (from 0.7 to 1.7 μm) and 2.5-fold increase in the surface area (from 24.2 to 60.1 %) compared to the planar surface (Fig. 7 and Table 1). Another increase in the temperature gradient to 55 K causes “over-etching” of samples after the standard time of etching (55 min), which can be seen in the Fig. 8c. This over-etching causes a drop in roughness (Table 2) and changes in UV/VIS reflectivity (Fig. 9). As can be seen in the Fig. 8 and Table 2, the considerable decrease in the etching time to 20 minutes with the temperature gradient of 55 K caused uneven surface etching and a relatively high etching speed (7.4 ± 0.5 μm). This means that the preparation of structured surfaces with a high ratio of secondary roughness requires precise control over the temperature gradient and etching time. The surface element composition was determined by the EDXA method. This analysis did not prove the presence of any interfering element, which confirmed that the etched surfaces consist entirely of pure silicon.
4. Conclusions The aim of this work was to demonstrate the possibility of using the temperature gradients and self-organized flow in the etching solution to modify the surface topography of the silicon wafers. It was found that the use of high temperature gradients and the optimal etching times can result in the preparation of specifically structured surfaces with low reflectivity in UVVIS spectrum as opposed to surfaces etched under constant temperature conditions. The creation of assymetrical pyramidal grains without the use of etching masks is caused by the generation of a secondary flow patterns in the etching solution in the specialy constructed apparatus. The presence of this specific flow pattern among pyramidal grains causes increased etching rate, which contributes to the creation of the specific pattern on the Si wafers surface.
Acknowledgments This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic – Program NPU I (LO1504). This research was also supported by grant of TBU No. IGA/FT/2015/014 and IGA/FT/2016/013 funded from the resources of specific university research.
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Vitae Jan Kotěna received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized functional layers for photovoltaic application. Antonín Minařík received the M.Sc. degree in 2004 and Ph.D. Degree in 2008 from the Faculty of technology, Tomas Bata University in Zlin, Czech Republic. His current research interests include study of self-organization processes and their possible application in technology of macromolecular compounds, for silicon etching and for micro / nano structured surfaces preparation. Erik Wrzecionko received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized functional layers and porous systems. Petr Smolka received the M.Sc. degree in 2003 and the Ph.D. degree in 2008 from the Faculty of Technology, Tomas Bata University, Zlin, Czech Republic. His current research interests include surface modification of polymers with low temperature plasma and thin/ultra-thin coating of plastics. Magda Minaříková received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized polymer surfaces. Martin Minařík received the M.Sc. degree in 2015 and now studies Ph.D. from the Faculty of Technology, Tomas Bata University, Zlin, Czech Republic His current Ph.D. research interests include surface modification with low temperature plasma and designing new types of plasma reactors. Aleš Mraček received the M.Sc. degree (Chemical Physics and Biophysics, Palacký University in Olomouc, Czech Republic) in 2000 and the Ph.D. degree (Technology of Macromolecular Substances, Tomas Bata University in Zlin, Czech Republic) in 2005. He currently works as associate professor and head of Physics and Materials Engineering Department at Tomas Bata University. His current research interests include surface physics and modification of polymer surfaces. Ivo Kuřitka received the M.Sc. degree (Faculty of Chemistry, Brno University of Technology, Czech Republic) in 2000 and the Ph.D. degree (Faculty of Chemistry, Brno University of Technology, Czech Republic) in 2003. He currently works as associate professor and senior researcher at the Centre of Polymer System, Tomas Bata University in Zlin. His current research interests include development of new multi-functional materials, sensors and actuators.
Michal Machovský received his scientific education in the Centre of Polymer Systems at Tomas Bata University in Zlin in 2013. His scientific interests are mainly focused on the synthesis of inorganics functional materials with magnetic, photocatalytic and antibacterial properties.
Figures captions Fig. 1. The schematic of the etching apparatus consisting of cooling and heating surfaces (1, 1´), sealed polymer cell (2), space for accumulation of the condensate (3), slanted condensation surface (4), sealing (5), temperature sensors (6, 6´), dosing inlet (7) sample (8), etching solution (9) and coolers (10, 10´). Fig. 2. The flow schematic of an etching solution at different temperature gradients. Fig. 3. The distribution and type of pyramidal grains prepared at ΔT = 0 K (a), ΔT = 20 K in the center (b) and ΔT = 20 K on the periphery (c) of the etched area indicated by a circular image cut (diameter 45 mm). Etching time 55 minutes. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 4. The schematic of etching process describing the heating and cooling plate, location of temperature sensors, etching solution, silicon wafer and temperature gradient direction.Fig. 5. SEM images of structures etched for 55 minutes in temperature gradient ΔT = 0 K (a), ΔT = 5 K (b), ΔT = 10 K (c), ΔT = 20 K (d). The silicon wafer temperature for all samples was kept constant at 353 K at the wafer/heating plate interface. Fig. 6. The measured reflectivity of Si surfaces etched for 55 minutes using 150 mm integrating sphere in temperature gradient ΔT = 0 K (a), ΔT = 5 K (b), ΔT = 10 K (c), ΔT = 20 K (d). Fig. 7. Surface topography of silicon samples etched for 55 minutes in temperature gradient ΔT = 5 K (a), ΔT = 10 K (b), ΔT = 20 K (c). The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 8. SEM images of silicon structures prepared in temperature gradient ΔT = 55 K after t = 20 minutes (a), t = 30 minutes (b), t = 55 minutes (c) of etching. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 9. The measured reflectivity using 150 mm integrating sphere of Si surfaces prepared in temperature gradient ΔT = 55 K after t = 20 minutes (a), t = 30 minutes (b), t = 55 minutes (c) of etching. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Table 1.The effect of temperature gradients on the variation of surface topography of anisotropically etched silicon wafers (100). Etching time for all samples was 55 minutes. The silicon wafer temperature was constant, 353 K at the wafer/heating plate interface.
Etching solution average temp.
Etching depth
Sa
Area ΔR 600 difference nm
(K)
(µm)
(µm)
(%)
(%)
ΔT = 0 K
351
2.3 ± 0.1
0.7
24.2
7.41
ΔT = 5 K
350
3.4 ± 0.1
1.2
30.0
7.70
ΔT = 10 K
348
9.6 ± 0.1
1.1
33.8
8.42
ΔT = 20 K
345
13.3 ± 0.1 1.7
60.1
10.47
Samples
Table 2. The influence of etching time on the variation of surface characteristics of anisotropically etched silicon wafers (100). Temperature gradient (T) for all samples was 55 K. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Average etching solution temperature for all samples was 338 K. Etching Samples
depth
Sa
Area difference
ΔR 600 nm
(µm)
(µm)
(%)
(%)
t = 20 min
7.4 ± 0.5
1.0
24.7
3.37
t = 30 min
17.7 ± 0.4
1.2
58.1
18.58
t = 55 min
23.2 ± 0.2
0.9
48.2
9.56
S0924-4247(16)30646-X http://dx.doi.org/doi:10.1016/j.sna.2017.05.019 SNA 10128
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
6-10-2016 5-5-2017 9-5-2017
Please cite this article as: Jan Kotˇena, Anton´ın Minaˇr´ık, Erik Wrzecionko, Petr Smolka, Magda Minaˇr´ıkov´a, Martin Minaˇr´ık, Aleˇs Mr´acˇ ek, Ivo Kuˇritka, Michal Machovsk´y, The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of temperature gradient on the variation of surface topography and reflectivity of anisotropically etched silicon wafers
Jan Kotěna†, Antonín Minařík*†,‡, Erik Wrzecionko†,‡, Petr Smolka†,‡, Magda Minaříková†,‡, Martin Minařík†,‡, Aleš Mráček†,‡, Ivo Kuřitka‡, Michal Machovský‡
†
Department of Physics and Materials Engineering, Tomas Bata University in Zlín, Vavrečkova 275, 760 01 Zlín, Czech Republic
‡
Centre of Polymer Systems, Tomas Bata University in Zlín, Třída Tomáše Bati 5678, 76001 Zlín, Czech Republic
* Corresponding author: E-mail address: [email protected]
Graphical abstract
Highlights
Anisotropic etching of silicon wafer under temperature gradient was studied.
The influence of temperature gradient magnitude and etching time was tested.
Novel etching method with standard etching solution.
High specific surface of the etched wafer resulting in dramatic reflectance decrease.
ABSTRACT A novel approach to wet etching of silicon wafers (100) is described in this study. Homogenous organized surface structures were prepared by the utilization of self-organized flow in the etching solution (Bénard-Marangoni thermocapillar instability). The driving force behind this process is a temperature gradient generated by the etching apparatus exclusively constructed for this research. The influences of temperature gradients (1 to 20 K) and etching time (20 to 55 min) were studied, with the potassium hydroxide / isopropyl alcohol etching solution. Obtained results indicate that self-organized flow can be utilized for specific and rapid etching, which allows significant variation of the topography and reflectivity of the silicon wafer surface textured in this way.
Keywords wet anisotropic etching, silicon, temperature gradients, Bénard-Marangoni thermocapillar instability, reflectivity.
1. Introduction The minimization of energy losses on silicon wafers intended for solar panel assembly has been studied from different perspectives by many researchers [1-11]. One of the critical parameters is the final reflectance of electromagnetic radiation from silicon surfaces modified either by texturing [1, 2, 5, 7, 9, 12-16] or by applying antireflective coating [2, 12]. Surface texturization is usually done by the controlled etching of monocrystalline (Miller indices – (100), (110), (111)) [3, 4, 6, 13, 17, 18] or polycrystalline [7] silicon wafers. For this purpose, acidic [19-21] or alkaline (KOH, NaOH, LiOH) [17] anisotropic solutions are widely used. The rate of this modification depends heavily on temperature [3, 4, 18], concentrations of reactants in the etching solution [3, 4, 6] and the presence of organic or inorganic additives [6, 16]. Addition of alcohol-based substances affects not only the etching rate, but also the homogeneity and distribution of the pyramidal grains [6, 13, 22]. On the other hand, different shapes of surface grains can also be obtained with the assistance of etching masks [9, 11, 14, 23-25]. These different approaches are usually evaluated based on their impact on environment and their costs [26]. Other important factors are the purity of
chemicals, pretreatment of the silicon wafers [26, 27] and circulation of the etching solution [28, 29]. Texturing itself takes place in single-purpose texturing machines, which are able to produce a large numbers of textured surfaces. These machines, however, often contain many moving parts, which is a potential source of contamination. Such problems could be minimized by utilizing forced circulation of the etching medium for effective silicon etching. Hence, the motivation for our investigation was to develop a relatively simple etching method utilizing temperature gradients and thus controlling the circulation of the etching solution. In this paper, we present an alternative etching method that utilizes Bénard-Marangoni convective thermocapillar instability [30, 31] in the etching solution for controllable modification of silicon wafers using temperature gradients. The above mentioned process can effectively and easily etch silicon surface in a similar manner as described in literature [3, 4, 17, 24]. Besides that, by controlling the etching process, various unique structures can be prepared [32]. The silicon wafer surface reflectivity has also been investigated in our study and was correlated with the morphological features of the prepared surfaces. The results have shown a significant improvement in surface reflectance compared with control surfaces prepared without temperature gradients.
2. Experimental section 2.1. Silicon surface modification. Single-crystalline boron-doped p-type silicon wafers (resistivity = 0.01 – 0.02 Ω.cm, radius 150 ± 0.2 mm, thickness 525 ± 15 µm, ON Semiconductor Czech Republic, LTD) were used for the preparation of uniform pyramidal texture. Initially, the wafers were cleaned according to the process described in the literature (RCA clean) [33]. After cleaning, the wafers were washed with pure ethanol and then blown dry with nitrogen. The wafers were then inserted into the etching cell of the apparatus preheated (10 minutes) to the desired temperature (353 K at the wafer/heating plate interface) and etched in a solution containing 2.5 w.% potassium hydroxide (Sigma-Aldrich, KOH basis ≥ 85%), 15 vol.% isopropyl alcohol (Lachema Brno, purity > 99.7 %) and ultra-pure H2O (resistivity > 18 MΩ.cm). Different temperature gradients (measured between wafer surface and a point located 35 mm above it, i. e. above the etching solution level) and etching times were used
in order to influence the final texture. When the etching was finished, the wafers were removed from the etching cell, washed with pure ethanol and blown dry using nitrogen
2.2. The characterization of surface changes. The topography of the etched surfaces was studied with the scanning electron microscope (SEM; -model FEI Nova NanoSEM 450). All SEM images presented in this study were obtained at 5 kV accelerating voltage with the back scattered electrons (BSE) detector in the immersion mode. The purity of presented surfaces was studied using energy dispersive X-Ray analysis (EDXA) method with the above-mentioned SEM. The topographic changes were characterized with the atomic force microscope (AFM; model Dimension ICON from Bruker) as well. All measurements were performed in the tapping mode with the MPP12120 probes, (Bruker). The sampling area was 40x40 μm with 512x512 resolution and 0.5 Hz scan speed. The topographic changes were also studied with the stylus profiler (Bruker – Dektak XT). Surface reflectivity was studied using spectrophotometer (model Perkin Elmer UV/VIS/NIR LAMBDA 1050) with 150 mm InGaAs integrating sphere..
2.3. The etching apparatus. The etching apparatus was assembled for the purpose of this study at the Tomas Bata University in Zlin and utilizes Bénard-Marangoni thermocappilar instability for controllable etching of silicon wafers. It consists of heating and cooling areas with evenly distributed temperatures, inert etching cell with sealing elements and a system for storing condensate, precise instruments for temperature regulation and separately placed ventilation system (Fig. 1). The main advantages are simplicity, the ability to etch with small volumes of etching solution and spontaneous movement of chemical products away from the etched surface. [34] The inner cell of the apparatus has the dimensions 60 x 40 mm (internal diameter x height) and can accommodate a silicon wafer 60 x 60 mm and etched area with diameter 44 mm. The overall dimensions of the apparatus are 150 x 150 x 350 mm (width x depth x height). Typical volume of the etching solution is about 10 ml.
3. Results and discussion The self-organized flow in the etching solution, Bénard-Marangoni thermocapillar instability, is driven through changes in surface tension, which causes temperature fluctuations on a phase boundary between liquid and gas [30, 31]. This flow can be characterized by the equation (1). dT d T Ma κ
(1)
Here, Ma stands for Marangoni number, determining the initiation of the BM-instability, the temperature difference between the bottom layer and an open surface dT, the thickness of the liquid layer d, dynamic viscosity µ and the coefficient of thermal diffusivity κ. The equation (1) shows that not only the absolute temperature, temperature gradient and the thickness of the liquid layer above the etched silicon wafer determine the creation and intensity of the self-organized flow. Our study focuses on the etching time and the temperature gradients. When the sample is subjected to minimal temperature gradient (Fig. 2a), the self-organized flow is not generated. The etching medium and products of the etching reaction fluctuate randomly over the silicon wafer surface, which is caused by Brownian motion and buoyancy forces, when warmer (less dense) liquid rises and colder descends. The Fig. 2b shows the self-organized flow caused by the temperature gradient. In this case, the etching solution is quickly changed over the surface, so no cumulation of reaction products occurs and no further measures their removal are needed as is described in literature [13]. Another increase in the temperature gradient causes secondary flow patterns to emerge among etched pyramidal structures (Fig. 2c). The assumptions for this hypothesis come from our experimental results and from the liquid flow mechanics [35]. The presence of the self-organized flow and its effect on the silicon wafer surface can result in uneven distribution of surface structures (from the perspective of size and type) when temperature gradient exceeds 10 K. As can be seen in the Fig. 3b and 3c, these inhomogeneities are presented on the edges of the etched area. This is caused by uneven distribution of surface tension and various forces near the edge, which was discussed in detail in our previous work focused on polymer systems, where similar inhomogeneity has been observed [36]. Although these inhomogeneities exist, they do not affect the final reflectance in the UV-VIS spectrum nor the etched area coverage, as discussed below. In addition, the etched surfaces were always characterized in the 12 cm2 central area, where these defects are
not present. When small temperature gradients are used (up to 10 K), all surfaces are covered with comparable pyramidal grains from the edge to the central area, as is shown in the circular cut in the Fig. 3a. However, these small temperature gradients do not cause the generation of the secondary flow patterns (Fig. 2c) and the creation of the secondary roughness (Fig. 3b and 3c). The Fig. 4 contains supplemental information to the apparatus, etching process and temperature gradient direction. In this experimental setup, the lower temperature was equal or higher, compared to the upper plate. Temperature gradient was defined as the temperature difference between lower and upper plate. The temperature difference was measured between the wafer/plate interface and a point located 35 mm above it, i. e. above the etching solution level. Also the average temperature of etching solution was monitored (Table 1 and Table 2). With increasing temperature gradient, the average solution temperature decreases (as the substrate temperature stays constant and the temperature of upper plate decreases). Even though the average solution temperature drops from 351 K to 338 K (Table 1 and Table 2), the etching rate increases (etch depth 13.3±0.1m and 23.2±0.2 m, respectively, during 55 min). This suggests dominating role of temperature gradient over the absolute temperature. Volatile components of the etching solution can condensate at the upper (relatively colder) plate, as it is separated from the solution surface with an air gap (the presence of a liquid/gas interface is one of the essential conditions for the occurrence of the Benard-Marangoni convection). This condensate partly reflux to the etching solution, partly stays trapped on the etching cell walls. The control experiments performed with pure KOH solution revealed that structures similar to those in the Fig. 3b etc. cannot be prepared without IPA. This fact suggests, that IPA stays in the etching solution during the whole etching process, though its concentration may decrease. Also, in any experiment the temperature of IPA boiling point (355 K) was not exceeded and the best results were obtained at the temperature 338 K (Fig. 8b). Indeed, the influence of etching solution concentration changes on the “hillocks” formations cannot be neglected [17, 32, 37-39], in our experiments though, it has minor effect compared to the temperature gradient and the organized liquid flow. The reflectance in the UV/VIS spectrum was measured due to the application potential of these surfaces in photovoltaics. For better clearness are the comparisons expressed as a reflectivity difference ΔR between the etched and the reference sample at the wavelength 600 nm (Fig. 6 and 9, Tables 1 and 2). The Fig. 5 demonstrates the effects of the temperature gradients as a driving force behind the variation in surface patterning. The Fig. 5a shows the patterned surface prepared without the
temperature gradient, which creates a surface commonly described in literature [3-5, 13, 17, 24]. Gradual increase in the temperature difference from 5 to 20 K causes changes in the flow mechanism (Fig. 2) and the transition from symmetrical patterns to asymmetrically etched surfaces (Fig. 5). These topographic changes directly influence the measured UV/VIS reflectivity (Fig. 6), which corresponds with the findings presented in the literature [1]. The increase in the temperature gradient value from 1 to 20 K causes more than 5-fold etching rate increase (from 2.3 ± 0.1 to 13.3 ± 0.2 μm) and more than 2.5-fold increase in the surface roughness Sa (from 0.7 to 1.7 μm) and 2.5-fold increase in the surface area (from 24.2 to 60.1 %) compared to the planar surface (Fig. 7 and Table 1). Another increase in the temperature gradient to 55 K causes “over-etching” of samples after the standard time of etching (55 min), which can be seen in the Fig. 8c. This over-etching causes a drop in roughness (Table 2) and changes in UV/VIS reflectivity (Fig. 9). As can be seen in the Fig. 8 and Table 2, the considerable decrease in the etching time to 20 minutes with the temperature gradient of 55 K caused uneven surface etching and a relatively high etching speed (7.4 ± 0.5 μm). This means that the preparation of structured surfaces with a high ratio of secondary roughness requires precise control over the temperature gradient and etching time. The surface element composition was determined by the EDXA method. This analysis did not prove the presence of any interfering element, which confirmed that the etched surfaces consist entirely of pure silicon.
4. Conclusions The aim of this work was to demonstrate the possibility of using the temperature gradients and self-organized flow in the etching solution to modify the surface topography of the silicon wafers. It was found that the use of high temperature gradients and the optimal etching times can result in the preparation of specifically structured surfaces with low reflectivity in UVVIS spectrum as opposed to surfaces etched under constant temperature conditions. The creation of assymetrical pyramidal grains without the use of etching masks is caused by the generation of a secondary flow patterns in the etching solution in the specialy constructed apparatus. The presence of this specific flow pattern among pyramidal grains causes increased etching rate, which contributes to the creation of the specific pattern on the Si wafers surface.
Acknowledgments This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic – Program NPU I (LO1504). This research was also supported by grant of TBU No. IGA/FT/2015/014 and IGA/FT/2016/013 funded from the resources of specific university research.
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Vitae Jan Kotěna received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized functional layers for photovoltaic application. Antonín Minařík received the M.Sc. degree in 2004 and Ph.D. Degree in 2008 from the Faculty of technology, Tomas Bata University in Zlin, Czech Republic. His current research interests include study of self-organization processes and their possible application in technology of macromolecular compounds, for silicon etching and for micro / nano structured surfaces preparation. Erik Wrzecionko received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized functional layers and porous systems. Petr Smolka received the M.Sc. degree in 2003 and the Ph.D. degree in 2008 from the Faculty of Technology, Tomas Bata University, Zlin, Czech Republic. His current research interests include surface modification of polymers with low temperature plasma and thin/ultra-thin coating of plastics. Magda Minaříková received the M.Sc. degree in 2014 at the Faculty of Technology, Tomas Bata University in Zlin, Czech Republic. His current Ph.D. research interests include preparation and characterization of hierarchically organized polymer surfaces. Martin Minařík received the M.Sc. degree in 2015 and now studies Ph.D. from the Faculty of Technology, Tomas Bata University, Zlin, Czech Republic His current Ph.D. research interests include surface modification with low temperature plasma and designing new types of plasma reactors. Aleš Mraček received the M.Sc. degree (Chemical Physics and Biophysics, Palacký University in Olomouc, Czech Republic) in 2000 and the Ph.D. degree (Technology of Macromolecular Substances, Tomas Bata University in Zlin, Czech Republic) in 2005. He currently works as associate professor and head of Physics and Materials Engineering Department at Tomas Bata University. His current research interests include surface physics and modification of polymer surfaces. Ivo Kuřitka received the M.Sc. degree (Faculty of Chemistry, Brno University of Technology, Czech Republic) in 2000 and the Ph.D. degree (Faculty of Chemistry, Brno University of Technology, Czech Republic) in 2003. He currently works as associate professor and senior researcher at the Centre of Polymer System, Tomas Bata University in Zlin. His current research interests include development of new multi-functional materials, sensors and actuators.
Michal Machovský received his scientific education in the Centre of Polymer Systems at Tomas Bata University in Zlin in 2013. His scientific interests are mainly focused on the synthesis of inorganics functional materials with magnetic, photocatalytic and antibacterial properties.
Figures captions Fig. 1. The schematic of the etching apparatus consisting of cooling and heating surfaces (1, 1´), sealed polymer cell (2), space for accumulation of the condensate (3), slanted condensation surface (4), sealing (5), temperature sensors (6, 6´), dosing inlet (7) sample (8), etching solution (9) and coolers (10, 10´). Fig. 2. The flow schematic of an etching solution at different temperature gradients. Fig. 3. The distribution and type of pyramidal grains prepared at ΔT = 0 K (a), ΔT = 20 K in the center (b) and ΔT = 20 K on the periphery (c) of the etched area indicated by a circular image cut (diameter 45 mm). Etching time 55 minutes. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 4. The schematic of etching process describing the heating and cooling plate, location of temperature sensors, etching solution, silicon wafer and temperature gradient direction.Fig. 5. SEM images of structures etched for 55 minutes in temperature gradient ΔT = 0 K (a), ΔT = 5 K (b), ΔT = 10 K (c), ΔT = 20 K (d). The silicon wafer temperature for all samples was kept constant at 353 K at the wafer/heating plate interface. Fig. 6. The measured reflectivity of Si surfaces etched for 55 minutes using 150 mm integrating sphere in temperature gradient ΔT = 0 K (a), ΔT = 5 K (b), ΔT = 10 K (c), ΔT = 20 K (d). Fig. 7. Surface topography of silicon samples etched for 55 minutes in temperature gradient ΔT = 5 K (a), ΔT = 10 K (b), ΔT = 20 K (c). The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 8. SEM images of silicon structures prepared in temperature gradient ΔT = 55 K after t = 20 minutes (a), t = 30 minutes (b), t = 55 minutes (c) of etching. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Fig. 9. The measured reflectivity using 150 mm integrating sphere of Si surfaces prepared in temperature gradient ΔT = 55 K after t = 20 minutes (a), t = 30 minutes (b), t = 55 minutes (c) of etching. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Table 1.The effect of temperature gradients on the variation of surface topography of anisotropically etched silicon wafers (100). Etching time for all samples was 55 minutes. The silicon wafer temperature was constant, 353 K at the wafer/heating plate interface.
Etching solution average temp.
Etching depth
Sa
Area ΔR 600 difference nm
(K)
(µm)
(µm)
(%)
(%)
ΔT = 0 K
351
2.3 ± 0.1
0.7
24.2
7.41
ΔT = 5 K
350
3.4 ± 0.1
1.2
30.0
7.70
ΔT = 10 K
348
9.6 ± 0.1
1.1
33.8
8.42
ΔT = 20 K
345
13.3 ± 0.1 1.7
60.1
10.47
Samples
Table 2. The influence of etching time on the variation of surface characteristics of anisotropically etched silicon wafers (100). Temperature gradient (T) for all samples was 55 K. The silicon wafer temperature for all samples was constant 353 K at the wafer/heating plate interface. Average etching solution temperature for all samples was 338 K. Etching Samples
depth
Sa
Area difference
ΔR 600 nm
(µm)
(µm)
(%)
(%)
t = 20 min
7.4 ± 0.5
1.0
24.7
3.37
t = 30 min
17.7 ± 0.4
1.2
58.1
18.58
t = 55 min
23.2 ± 0.2
0.9
48.2
9.56