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A simple and robust model to explain convex corner undercutting in wet bulk micromachining

A simple and robust model to explain convex corner undercutting in wet bulk micromachining In this paper, a simple and robust model is presented to explain the main reason behind undercutting at convex corners and no-undercutting at concave corners. The etch rate of the tangent plane at convex corner and the role of dangling bond in etching process are utilized to explain the undercutting at convex corner and the no-undercutting at concave corner, respectively. The present model shows that {110} is the tangent plane at convex corner which exhibits higher etch rate than the neighboring {111} plane in all types of anisotropic etchants; consequently the undercutting occurs at convex corners. The absence of dangling bonds at concave corner prevents the undercutting there. Moreover, the same model explains the reason of very less undercutting when the etching is carried out in surfactant-added tetramethylammonium hydroxide (TMAH). Keywords: Convex and concave corners; MEMS; Silicon; Wet anisotropic etching; KOH; TMAH Introduction reduces dramatically when a very small amount (e.g. 0.1% The wafer manufacturing industries commonly produce by volume) of surfactant (e.g. Triton-X-100, polyethylene the silicon wafers with three principle orientations namely glycol (PEG), NC-200, etc.) is added in the etchant {111}, {110} and {100}. Out of these three orientations, [16-23]. In order to explain the mechanism behind the {100} silicon wafers are most widely employed for the corner undercutting, several models have been proposed fabrication of microelectromechanical systems (MEMS) [12,24-27]. They explain that the appearance of high index and complementary metal oxide semiconductor (CMOS) planes during etching is the main cause of the undercut- devices. In the fabrication of MEMS, alkaline solution ting. However, these models do not explain very clearly (e.g. potassium hydroxide (KOH), tetramethylammonium why the undercutting starts at convex corner and why not hydroxide (TMAH), etc.) based silicon anisotropic etching at concave corners. is frequently used to make a wide range of microstructures In this paper, a simple and robust model is proposed in silicon wafers [1-10]. In this etching method, as shown to explain the phenomenon of severe undercutting at in Figure 1, undercutting occurs at mask patterns the convex corner as well as no-undercutting at the con- containing the extruded (or convex) corners [11]. The cave corner in wet anisotropic etchants. Moreover, the shape of the corner obtained after undercutting is deter- same model presents why the undercutting is reduced in mined using a polar diagram of lateral underetch rates as surfactant added TMAH solution. illustrated in Figure 1 [12]. The corner undercutting also takes place on {110} silicon wafers [13-15]. On one hand, Findings undercutting is advantageously used for the releasing of Figure 2 shows the schematic view of different planes in microstructures (e.g. cantilever beam), but on the other a unit cell and the shape of mesa structure fabricated on hand, it is undesirable for the realization of mesa struc- Si{100} wafer surface. The concave and convex corners tures, bent V-grooves, proof mass for accelerometer, etc. in a microstructure are illustrated in Figure 3. The {111} In the TMAH-based anisotropic etchants, undercutting planes are the most stable (i.e. lowest etch rate) planes in wet anisotropic etchants. However the convex corners (i.e. the intersection of the two {111} planes) is still vul- * Correspondence: prem@iith.ac.in nerable to etching and this vulnerability is the reason for MEMS and Micro/Nano Systems Laboratory, Department of Physics, Indian Institute of Technology Hyderabad, Hyderabad, India extensive undercutting. Now the question is, why the © 2013 Pal and Singh; licensee Springer. This is an Open A ccess article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 2 of 6 http://www.mnsl-journal.com/content/1/1/1 Figure 1 Schematic and SEM pictures of square shaped mask pattern etched in anisotropic etchant on {100} Si wafer. Undercutting shape is analyzed using theorientation-dependent lateral underetchrates that are commonly determined by wagon-wheel method [12]. intersecting {111} planes at the convex corner are vul- rate of the corner is much higher than the correspond- nerable? The proposed undercut model answers it on ing {111} planes forming the corner and therefore the the basis of the fact that the tangent plane on the phenomenon of undercutting is observed at the convex intersecting {111} planes (i.e. the convex corner) is {110} corners. It is to be noted here that the concept of dangling as shown in the Figure 2 and this plane exhibits high bond is not enough to explain the variation in etching as etch rate in pure KOH and TMAH solutions [7,10,11]. both the convex corner and the {111} plane contain the Since the convex corner lies on the {110} plane, the etch atoms with only one dangling bond as can be observed in Figure 2 Schematic view: (a) a unit cell exhibiting three principle planes, (b) {110} tangent plane on a convex corner of a mesa structure formed by four convex corners on Si{100} surface, (c) convex corner and its tangent plane in a unit cell. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 3 of 6 http://www.mnsl-journal.com/content/1/1/1 Figure 3 Schematic representation of concave and convex corners: (a) masking pattern on {100}Si wafer, (b) anisotropically etched pattern assuming no-undercutting takes place at convex corners, (c) close-up view of concave corner exhibiting silicon atoms of {111} plane with one dangling bond, while the atoms belonging to concave corner do not contain any dangling bonds, (d) close-up view of convex corner showing silicon atoms of {111} plane and the convex corner with one dangling bond. Figure 3(d). The etching behavior of tangent plane at con- reason behind this is that the crystallographic structure of vex corner is more appropriate to describe the etching silicon arranges the atoms at the two types of corners (i.e. characteristic of convex corner in anisotropic etchants. convex and concave) in such a way that the convex edged One more interesting question which arises here is that silicon atoms have one dangling bond, while the concave why there is no undercutting observed at the concave cor- cornered silicon atoms have all their bonds engaged and ner, even though these corners are also formed by the thus there is no dangling bond. The absence of the dan- intersection of two {111} planes. If we closely observe the gling bond at the concave corner restricts undercutting, concave and convex corners (Figure 3), we can easily no- however the etching occurs parallel to the {111} planes. tice the difference in the bond structure at the intersecting This is a very simple way of explaining why a convex cor- edge. The silicon atoms belonging to a convex edge con- ner is more vulnerable to undercutting whereas there is sist of dangling bonds (Figure 3(d)), while the atoms be- no-undercutting at the concave corner. longing to the concave corner (Figure 3(c)) do not contain Now the last concern is, why does the undercutting at any dangling bond (i.e. all the bonds are engaged). The convex corner decrease dramatically in surfactant-added Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 4 of 6 http://www.mnsl-journal.com/content/1/1/1 TMAH [16-23]. The summary of the etching character- reduced significantly when etching is carried out in istics of the pure and very small amount of surfactant Triton-X-100 (i.e. surfactant) added TMAH. (0.1% by volume) added TMAH solution is presented in The surfactant molecules form an adsorbed monolayer Figure 4. The etch rate of {100}Si is almost the same in film on the surface with the hydrophobic part of the pure and surfactant-added TMAH (Figure 4(a), while molecules (or head) in contact with the hydrophobic the etch rate of {110} silicon is decreased to a consider- surface while the hydrophilic part of the molecules (or ably low level when a very small amount of surfactant is tail) remains in contact with water [22,28]. Hence the added into the etchant (Figure 4(b)). As can be seen in surfactant molecules adsorb more densely on more Figures 4(c)-4(e), the phenomenon of undercutting is hydrophobic surface. The relative hydrophobicity of the Figure 4 Etching characteristics of pure and surfactant (Triton X-100) added 25 wt% TMAH: (a) Si{100} etch rate at different temperatures; (b) Si{110} etch rate at different temperatures; (c) etched profiles of convex corners; (d) undercutting (U= l/d) at convex corners. (e) Micromachining of alphabets in {100}Si surface using (i) pure TMAH and (ii) TMAH+Triton. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 5 of 6 http://www.mnsl-journal.com/content/1/1/1 silicon surface can be estimated by the density of H- on the presence of {110} tangent plane at the convex terminations. The Si{110} is more hydrophobic (lower corner i.e. the silicon atoms of convex corner belong to H-density) than Si{110}, resulting in the formation of a {110} plane. As shown in Figure 5, the surfactant mole- more densely packed surfactant layer [22]. Several stud- cules form a dense layer on the convex corner. This ies have been performed to confirm the orientation dense layer inhibits the etchant to react chemically with dependent adsorption of surfactant molecules [18,19,22]. the silicon atoms at the corner that results in dramatic They explain that the maximum adsorption is for {111} reduction in the undercutting. It means that when the surface followed by {110} and {100} surfaces. It can be surfactant molecules adsorb very densely, the surface observed from Figure 4(a) that the etch rate of {100} is area available for the etchant to react reduces signifi- almost unaffected when the surfactant is incorporated in cantly and thus undercutting rate is remarkably the etchant. It means that the layer of surfactant mole- suppressed. On the other hand, the {100} plane is not able cules on {100} surface is not able to protect the surface to attract the surfactant molecules more compactly owing from the etchant and thus etch rate is almost unaffected. to its less hydrophobicity and thus the etchant can easily In the case of Si{110} (Figure 4(b)), significant reduction react chemically with the silicon atoms and leads to in the etch rate indicates that the adsorbed layer of sur- almost the same etch rate as shown in Figure 4(a). factant molecules partially protects the surface from the direct attack of reactants during etching. Conclusions In order to emphasize the effect of undercutting on A simple model is proposed to explain the etching char- the resultant shape of the fabricated structure, the SEM acteristics of convex and concave corners on Si{110} pictures of alphabets “IITH”, which comprise concave surface in wet anisotropic etchants. The proposed model and convex corners, etched in pure and surfactant- explains the basic reasons behind the corner undercut- added TMAH are shown in Figure 4(e)(i) and 4(e)(ii), re- ting in a very simple and robust way. Moreover, it ex- spectively. The letters micromachined in pure TMAH plains the dramatic reduction in the undercutting in have lost their shapes, while they retain their shapes surfactant-added TMAH solution. In the case of the when etching is performed in surfactant added TMAH. surfactant-added TMAH, crystallographic orientation This kind of etching behavior can be easily understood dependent adsorption of the surfactant molecules (owing through the proposed undercutting model as it is based to varying hydrophobicity of the crystallographic planes) Figure 5 Schematic representation of surfactant adsorption on silicon atoms of convex corners, which belong to {110} planes, in surfactant-added TMAH solution during etching process. The dense layer of surfactant molecules protect the convex corners from etchant that result in the reduction of undercutting. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 6 of 6 http://www.mnsl-journal.com/content/1/1/1 is employed to describe the significant reduction in the 14. Kim B, Cho DD (1998) Aqueous KOH etching of silicon (110) etch characteristics and compensation methods for convex corners. undercutting at convex corners. In order to explain the J Electrochem Soc 145(7):2499–2508. doi:10.1149/1.1838668 reason of no-undercutting at concave corners, the role 15. Dong W, Zhang X, Liu C, Li M, Xu B, Chen W (2004) Mechanism for convex of dangling bonds in etching mechanism is exploited. corner undercutting of (110) silicon in KOH. Microelectronics J 35:417–419, http://dx.doi.org/10.1016/j.mejo.2004.01.005 The undercutting at convex corners is explained using the 16. Yang CR, Chen PY, Yang CH, Chiou YC, Lee RT (2005) Effects of various etch rate behavior of tangent plane appearing at this type ion-typed surfactants on silicon anisotropic etching properties in KOH and of corners. TMAH solutions. Sens Actuators A15:271–281. doi:10.1016/j.sna.2004.09.017 17. Sekimura M (1999) Anisotropic etching of surfactant-added TMAH solution. In: Proc. 12th IEEE Micro-Electro-Mech. Syst. Conf. , Orlando, Florida, pp 650–655. doi:10.1109/MEMSYS.1999.746904 Competing interests 18. Gosálvez MA, Tang B, Pal P, Sato K, Kimura Y, Ishibashi K (2009) Orientation The authors declare that they have no competing interests. and concentration dependent surfactant adsorption on silicon in aqueous alkaline solutions: explaining the changes in the etch rate, roughness and Authors’ contributions undercutting for MEMS applications. J Micromech Microeng 19:125011 PP performed the experiments, analyzed the experimental results and (18pp). http://www.sciencedirect.com/science/article/pii/ prepared the manuscript. SSS developed 3-D and 2-D models for better S0924424700003174 visualization of theory and contributed significantly during the preparation of 19. Tang B, Pal P, Gosalvez MA, Shikida M, Sato K, Amakawa H, Itoh S (2009) manuscript. All authors read and approved the final manuscript. Ellipsometry study of the adsorbed surfactant thickness on Si{110} and Si {100} and the effect of pre-adsorbed surfactant layer on etching Acknowledgments characteristics in TMAH. Sens Actuators A156:334–341. doi:10.1016/j. This work was supported by research grant from the Department of Science sna.2009.10.017 and Technology (Project No. SR/S3/MERC/072/2011), New Delhi, India. 20. Resnik D, Vrtacnik D, Aljancic U, Mozek M, Amon S (2005) The role of Triton surfactant in anisotropic etching of {110} reflective planes on (100) silicon. Received: 10 March 2013 Accepted: 27 May 2013 J Micromech Microeng 15:1174–1183. doi:10.1088/0960-1317/15/6/007 Published: 6 August 2013 21. Sarro PM, Brida D, van der Vlist W, Brida S (2000) Effect of surfactant on surface quality of silicon microstructures etched in saturated TMAHW solutions. References Sens Actuators A 85:340–345, http://dx.doi.org/10.1016/S0924-4247(00)00317-4 1. Kang I, Haskard MR, Samaan ND (1997) A study of two-step silicon 22. Pal P, Sato K, Gosalvez MA, Kimura Y, Ishibashi K, Niwano M, Hida H, Tang B, anisotropic etching for a polygon-shaped microstructure using KOH Itoh S (2009) Surfactant adsorption on single crystal silicon surfaces in solution. Sens Actuators A62:646–651, http://dx.doi.org/10.1016/S0924-4247 TMAH solution: orientation-dependent adsorption detected by In-situ (97)01500-8 Infra-Red spectroscopy. J Microelectromech Syst 18:1345–1356. doi:10.1109/ 2. Jiang Y, Liu G, Zhou J (2009) A novel process for circle-like 3D JMEMS.2009.2031688 microstructures by two-step wet etching. J Micromech Microeng 23. Pal P, Sato K, Gosalvez MA (2012) Etched profile control in anisotropic 19015005:5. doi:10.1088/0960-1317/19/1/015005 etching of silicon by TMAH+Triton. J Micromech Microeng 22(6):065013 3. Kwon JW, Kim ES (2002) Multi-level microfluidic channel routing with (9pp). doi:10.1088/0960-1317/22/6/065013 protected convex corners. Sens Actuators A97-98:729–733. doi:10.1016/ 24. Chahoud M, Wehmann HH, Schlachetzki A (1998) Etching simulation of S0924-4247(02)00012-2 convex and mixed InP and Si structures. Sens Actuators A69:251–258. http:// 4. Rao MP, Aimi MF, MacDonald NC (2004) Single-mask, three-dimensional www.sciencedirect.com/science/article/pii/S0924424702000171 microfabrication of high-aspect-ratio structures in bulk silicon using reactive 25. Schroder H, Obermeier E (2000) A new model for Si convex corner {100} ion etching lag and sacrificial oxidation. Appl Phys Lett 85:6281–6283, undercutting in anisotropic KOH etching. J Micromech Microeng 10:163–170. http://dx.doi.org/10.1063/1.1834720 doi:10.1088/0960-1317/10/2/311 5. Lee S, Park S, Cho D (1999) The surface/bulk micromachining (SBM) process: 26. Shikida M, Nanbara K, Koizumi T, Sasaki H, Sato K, Odagaki M, Ando M, Furuta a new method for fabricating released microelectromechanical systems in S, Asaumi K (2002) A model explaining mask-corner undercut phenomena in single crystal silicon. J Microelectromech Syst 8:409–416. doi:10.1109/ anisotropic silicon etching: a saddle point in the etching-rate diagram. Sens 84.809055 Actuators A 97–98:758–763, http://dx.doi.org/10.1016/S0924-4247(02)00017-1 6. Chu HY, Fang W (2004) A vertical convex corner compensation and non {111} 27. Chang Chien WT, Chang CO, Lo YC, Li ZW, Chou CS (2005) On the Miller-indices crystal planes protection for wet anisotropic bulk micromachining process. determination of Si convex corner undercut planes. J Micromech Microeng {100} J Micromech Microeng 14:806–813. doi:10.1088/0960-1317/14/6/007 15:833–842. doi:10.1088/0960-1317/15/4/022 7. Zubel I, Kramkowska M (2005) Possibilities of extension of 3D shapes by 28. Tiberg F (1996) Physical characterization of non-ionic surfactant layers bulk micromachining of different Si (hk l) substrates. J Micromech adsorbed at hydrophilic and hydrophobic solid surfaces by time-resolved Microeng 15:485–493. doi:10.1088/0960-1317/15/3/008 ellipsometry. J Chem Soc Faraday Trans 92:531–538. doi:10.1039/ 8. 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Trieu HK, Mokwa W (1998) A generalized model describing corner undercutting by the experimental analysis of TMAH/IPA. J Micromech Microeng 8:80–83. doi:10.1088/0960-1317/8/2/009 13. Jia C, Dong W, Liu C, Zhang X, Zhou J, Zhong Z, Xue H, Zang H, Xu B, Chen W (2006) Convex corners undercutting and rhombus compensation in KOH with and without IPA solution on (110) silicon. Microelectronics J 37:1297–1301. doi:10.1016/j.mejo.2006.07.008 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

A simple and robust model to explain convex corner undercutting in wet bulk micromachining

Micro and Nano Systems Letters , Volume 1 (1) – Aug 6, 2013

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Springer Journals
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Copyright © 2013 by Pal and Singh; licensee Springer.
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Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
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Abstract

In this paper, a simple and robust model is presented to explain the main reason behind undercutting at convex corners and no-undercutting at concave corners. The etch rate of the tangent plane at convex corner and the role of dangling bond in etching process are utilized to explain the undercutting at convex corner and the no-undercutting at concave corner, respectively. The present model shows that {110} is the tangent plane at convex corner which exhibits higher etch rate than the neighboring {111} plane in all types of anisotropic etchants; consequently the undercutting occurs at convex corners. The absence of dangling bonds at concave corner prevents the undercutting there. Moreover, the same model explains the reason of very less undercutting when the etching is carried out in surfactant-added tetramethylammonium hydroxide (TMAH). Keywords: Convex and concave corners; MEMS; Silicon; Wet anisotropic etching; KOH; TMAH Introduction reduces dramatically when a very small amount (e.g. 0.1% The wafer manufacturing industries commonly produce by volume) of surfactant (e.g. Triton-X-100, polyethylene the silicon wafers with three principle orientations namely glycol (PEG), NC-200, etc.) is added in the etchant {111}, {110} and {100}. Out of these three orientations, [16-23]. In order to explain the mechanism behind the {100} silicon wafers are most widely employed for the corner undercutting, several models have been proposed fabrication of microelectromechanical systems (MEMS) [12,24-27]. They explain that the appearance of high index and complementary metal oxide semiconductor (CMOS) planes during etching is the main cause of the undercut- devices. In the fabrication of MEMS, alkaline solution ting. However, these models do not explain very clearly (e.g. potassium hydroxide (KOH), tetramethylammonium why the undercutting starts at convex corner and why not hydroxide (TMAH), etc.) based silicon anisotropic etching at concave corners. is frequently used to make a wide range of microstructures In this paper, a simple and robust model is proposed in silicon wafers [1-10]. In this etching method, as shown to explain the phenomenon of severe undercutting at in Figure 1, undercutting occurs at mask patterns the convex corner as well as no-undercutting at the con- containing the extruded (or convex) corners [11]. The cave corner in wet anisotropic etchants. Moreover, the shape of the corner obtained after undercutting is deter- same model presents why the undercutting is reduced in mined using a polar diagram of lateral underetch rates as surfactant added TMAH solution. illustrated in Figure 1 [12]. The corner undercutting also takes place on {110} silicon wafers [13-15]. On one hand, Findings undercutting is advantageously used for the releasing of Figure 2 shows the schematic view of different planes in microstructures (e.g. cantilever beam), but on the other a unit cell and the shape of mesa structure fabricated on hand, it is undesirable for the realization of mesa struc- Si{100} wafer surface. The concave and convex corners tures, bent V-grooves, proof mass for accelerometer, etc. in a microstructure are illustrated in Figure 3. The {111} In the TMAH-based anisotropic etchants, undercutting planes are the most stable (i.e. lowest etch rate) planes in wet anisotropic etchants. However the convex corners (i.e. the intersection of the two {111} planes) is still vul- * Correspondence: prem@iith.ac.in nerable to etching and this vulnerability is the reason for MEMS and Micro/Nano Systems Laboratory, Department of Physics, Indian Institute of Technology Hyderabad, Hyderabad, India extensive undercutting. Now the question is, why the © 2013 Pal and Singh; licensee Springer. This is an Open A ccess article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 2 of 6 http://www.mnsl-journal.com/content/1/1/1 Figure 1 Schematic and SEM pictures of square shaped mask pattern etched in anisotropic etchant on {100} Si wafer. Undercutting shape is analyzed using theorientation-dependent lateral underetchrates that are commonly determined by wagon-wheel method [12]. intersecting {111} planes at the convex corner are vul- rate of the corner is much higher than the correspond- nerable? The proposed undercut model answers it on ing {111} planes forming the corner and therefore the the basis of the fact that the tangent plane on the phenomenon of undercutting is observed at the convex intersecting {111} planes (i.e. the convex corner) is {110} corners. It is to be noted here that the concept of dangling as shown in the Figure 2 and this plane exhibits high bond is not enough to explain the variation in etching as etch rate in pure KOH and TMAH solutions [7,10,11]. both the convex corner and the {111} plane contain the Since the convex corner lies on the {110} plane, the etch atoms with only one dangling bond as can be observed in Figure 2 Schematic view: (a) a unit cell exhibiting three principle planes, (b) {110} tangent plane on a convex corner of a mesa structure formed by four convex corners on Si{100} surface, (c) convex corner and its tangent plane in a unit cell. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 3 of 6 http://www.mnsl-journal.com/content/1/1/1 Figure 3 Schematic representation of concave and convex corners: (a) masking pattern on {100}Si wafer, (b) anisotropically etched pattern assuming no-undercutting takes place at convex corners, (c) close-up view of concave corner exhibiting silicon atoms of {111} plane with one dangling bond, while the atoms belonging to concave corner do not contain any dangling bonds, (d) close-up view of convex corner showing silicon atoms of {111} plane and the convex corner with one dangling bond. Figure 3(d). The etching behavior of tangent plane at con- reason behind this is that the crystallographic structure of vex corner is more appropriate to describe the etching silicon arranges the atoms at the two types of corners (i.e. characteristic of convex corner in anisotropic etchants. convex and concave) in such a way that the convex edged One more interesting question which arises here is that silicon atoms have one dangling bond, while the concave why there is no undercutting observed at the concave cor- cornered silicon atoms have all their bonds engaged and ner, even though these corners are also formed by the thus there is no dangling bond. The absence of the dan- intersection of two {111} planes. If we closely observe the gling bond at the concave corner restricts undercutting, concave and convex corners (Figure 3), we can easily no- however the etching occurs parallel to the {111} planes. tice the difference in the bond structure at the intersecting This is a very simple way of explaining why a convex cor- edge. The silicon atoms belonging to a convex edge con- ner is more vulnerable to undercutting whereas there is sist of dangling bonds (Figure 3(d)), while the atoms be- no-undercutting at the concave corner. longing to the concave corner (Figure 3(c)) do not contain Now the last concern is, why does the undercutting at any dangling bond (i.e. all the bonds are engaged). The convex corner decrease dramatically in surfactant-added Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 4 of 6 http://www.mnsl-journal.com/content/1/1/1 TMAH [16-23]. The summary of the etching character- reduced significantly when etching is carried out in istics of the pure and very small amount of surfactant Triton-X-100 (i.e. surfactant) added TMAH. (0.1% by volume) added TMAH solution is presented in The surfactant molecules form an adsorbed monolayer Figure 4. The etch rate of {100}Si is almost the same in film on the surface with the hydrophobic part of the pure and surfactant-added TMAH (Figure 4(a), while molecules (or head) in contact with the hydrophobic the etch rate of {110} silicon is decreased to a consider- surface while the hydrophilic part of the molecules (or ably low level when a very small amount of surfactant is tail) remains in contact with water [22,28]. Hence the added into the etchant (Figure 4(b)). As can be seen in surfactant molecules adsorb more densely on more Figures 4(c)-4(e), the phenomenon of undercutting is hydrophobic surface. The relative hydrophobicity of the Figure 4 Etching characteristics of pure and surfactant (Triton X-100) added 25 wt% TMAH: (a) Si{100} etch rate at different temperatures; (b) Si{110} etch rate at different temperatures; (c) etched profiles of convex corners; (d) undercutting (U= l/d) at convex corners. (e) Micromachining of alphabets in {100}Si surface using (i) pure TMAH and (ii) TMAH+Triton. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 5 of 6 http://www.mnsl-journal.com/content/1/1/1 silicon surface can be estimated by the density of H- on the presence of {110} tangent plane at the convex terminations. The Si{110} is more hydrophobic (lower corner i.e. the silicon atoms of convex corner belong to H-density) than Si{110}, resulting in the formation of a {110} plane. As shown in Figure 5, the surfactant mole- more densely packed surfactant layer [22]. Several stud- cules form a dense layer on the convex corner. This ies have been performed to confirm the orientation dense layer inhibits the etchant to react chemically with dependent adsorption of surfactant molecules [18,19,22]. the silicon atoms at the corner that results in dramatic They explain that the maximum adsorption is for {111} reduction in the undercutting. It means that when the surface followed by {110} and {100} surfaces. It can be surfactant molecules adsorb very densely, the surface observed from Figure 4(a) that the etch rate of {100} is area available for the etchant to react reduces signifi- almost unaffected when the surfactant is incorporated in cantly and thus undercutting rate is remarkably the etchant. It means that the layer of surfactant mole- suppressed. On the other hand, the {100} plane is not able cules on {100} surface is not able to protect the surface to attract the surfactant molecules more compactly owing from the etchant and thus etch rate is almost unaffected. to its less hydrophobicity and thus the etchant can easily In the case of Si{110} (Figure 4(b)), significant reduction react chemically with the silicon atoms and leads to in the etch rate indicates that the adsorbed layer of sur- almost the same etch rate as shown in Figure 4(a). factant molecules partially protects the surface from the direct attack of reactants during etching. Conclusions In order to emphasize the effect of undercutting on A simple model is proposed to explain the etching char- the resultant shape of the fabricated structure, the SEM acteristics of convex and concave corners on Si{110} pictures of alphabets “IITH”, which comprise concave surface in wet anisotropic etchants. The proposed model and convex corners, etched in pure and surfactant- explains the basic reasons behind the corner undercut- added TMAH are shown in Figure 4(e)(i) and 4(e)(ii), re- ting in a very simple and robust way. Moreover, it ex- spectively. The letters micromachined in pure TMAH plains the dramatic reduction in the undercutting in have lost their shapes, while they retain their shapes surfactant-added TMAH solution. In the case of the when etching is performed in surfactant added TMAH. surfactant-added TMAH, crystallographic orientation This kind of etching behavior can be easily understood dependent adsorption of the surfactant molecules (owing through the proposed undercutting model as it is based to varying hydrophobicity of the crystallographic planes) Figure 5 Schematic representation of surfactant adsorption on silicon atoms of convex corners, which belong to {110} planes, in surfactant-added TMAH solution during etching process. The dense layer of surfactant molecules protect the convex corners from etchant that result in the reduction of undercutting. Pal and Singh Micro and Nano Systems Letters 2013, 1:1 Page 6 of 6 http://www.mnsl-journal.com/content/1/1/1 is employed to describe the significant reduction in the 14. Kim B, Cho DD (1998) Aqueous KOH etching of silicon (110) etch characteristics and compensation methods for convex corners. undercutting at convex corners. In order to explain the J Electrochem Soc 145(7):2499–2508. doi:10.1149/1.1838668 reason of no-undercutting at concave corners, the role 15. Dong W, Zhang X, Liu C, Li M, Xu B, Chen W (2004) Mechanism for convex of dangling bonds in etching mechanism is exploited. corner undercutting of (110) silicon in KOH. Microelectronics J 35:417–419, http://dx.doi.org/10.1016/j.mejo.2004.01.005 The undercutting at convex corners is explained using the 16. Yang CR, Chen PY, Yang CH, Chiou YC, Lee RT (2005) Effects of various etch rate behavior of tangent plane appearing at this type ion-typed surfactants on silicon anisotropic etching properties in KOH and of corners. TMAH solutions. Sens Actuators A15:271–281. doi:10.1016/j.sna.2004.09.017 17. Sekimura M (1999) Anisotropic etching of surfactant-added TMAH solution. In: Proc. 12th IEEE Micro-Electro-Mech. Syst. Conf. , Orlando, Florida, pp 650–655. doi:10.1109/MEMSYS.1999.746904 Competing interests 18. Gosálvez MA, Tang B, Pal P, Sato K, Kimura Y, Ishibashi K (2009) Orientation The authors declare that they have no competing interests. and concentration dependent surfactant adsorption on silicon in aqueous alkaline solutions: explaining the changes in the etch rate, roughness and Authors’ contributions undercutting for MEMS applications. J Micromech Microeng 19:125011 PP performed the experiments, analyzed the experimental results and (18pp). http://www.sciencedirect.com/science/article/pii/ prepared the manuscript. SSS developed 3-D and 2-D models for better S0924424700003174 visualization of theory and contributed significantly during the preparation of 19. Tang B, Pal P, Gosalvez MA, Shikida M, Sato K, Amakawa H, Itoh S (2009) manuscript. All authors read and approved the final manuscript. Ellipsometry study of the adsorbed surfactant thickness on Si{110} and Si {100} and the effect of pre-adsorbed surfactant layer on etching Acknowledgments characteristics in TMAH. Sens Actuators A156:334–341. doi:10.1016/j. This work was supported by research grant from the Department of Science sna.2009.10.017 and Technology (Project No. SR/S3/MERC/072/2011), New Delhi, India. 20. 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