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Maskless pattern fabrication on Pyrex 7740 glass surface by using nano-scratch with HF wet etching
Scripta Materialia 52 (2005) 117–122 www.actamat-journals.com
Maskless pattern fabrication on Pyrex 7740 glass surface by using nano-scratch with HF wet etching S.W. Youn a
a,*
, C.G. Kang
b,1
Department of Precision and Mechanical Engineering, Pusan National University, San 30, JangJeon-Dong, Kumjung-Ku, P/Busan 609735, Korea b School of Mechanical Engineering, Pusan National University, P/Busan 609-735, Korea Received 19 March 2004; received in revised form 7 September 2004; accepted 17 September 2004
Abstract Line patterns were machined on borosilicate (Pyrex 7740 glass) surface using the combination of mechanical machining by Nanoindenter XP and HF wet etching, and a etch-mask effect of the affected layer of the nano-scratched Pyrex 7740 glass surface was investigated. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Lithography; Nano-scratch test; Silicate
1. Introduction The need for a low-priced patterning technique of hard-brittle materials such as silicon and glass has increased as a result of the development of NEMS/MEMS parts such as chromatography chips, lab-on-a-chip (LOC), system-on-chip (SOC), storage medias, opticallenses, filters, and others [1,2]. Hard-brittle materials such as silicon, glass and ceramics have two modes of materials removal, ductile and brittle. It is well known that a no-crack and fine machined surface can be achieved by a ductile-regime machining. Materials removal with plastic deformation, known as ductileregime machining, has been achieved by a very shallow cut. When the depth or the force of a cut exceeds some threshold value (i.e. critical depth or force) the material removal mode will change from ductile mode to brittle *
Corresponding author. Tel.: +82 51 510 1455; fax: +82 51 512 1722. E-mail addresses: [email protected] (S.W. Youn), cgkang@ pusan.ac.kr (C.G. Kang). 1 Tel.: +82 51 510 2335.
mode [3–5] The ductile-regime machining concept has been used in nanoprobes based on lithographic technology such as static/dynamic scratches using AFM and FFM [6–11]. A nano-scratch technique has been recognized as a potential maskless nanofabrication technique that can substitute for or supplement optical lithography because of its operational versatility, low costs for initial facilities and manufacture, simplicity of process, and material selectivity. There is an additional advantage that the size of structures can be varied by controlling the normal load applied to a single asperity tip that serves as the cutting tool. However, static/dynamic scratches themselves are not suitable for mass production because they are time-consuming methods and are not economical for commercial applications. One solution is to fabricate a mold that will be used for mass production processes such as nanoimprint, PDMS rheology casting, and others. It is important to fabricate a mold which has pattern structures over a large area of surface because the key advantages of the nanoimprint and the PDMS casting process are the ability to pattern nano/micro structures with a high-throughput and low cost [12–14].
1359-6462/$ - see front matter 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.09.016
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S.W. Youn, C.G. Kang / Scripta Materialia 52 (2005) 117–122
From this point of view, we believe that the scratch experiment using a nanoindenter is one of the most promising possibilities for developing a nanomachining tool. Nanoindenters are often used for the measurement of mechanical properties such as hardness, elastic modulus, coefficient of friction, and lateral force of very thin films in any direction. Also, the amount of elastic recovery of materials can be measured from the cross profile topography and residual morphology profile graph. These mechanical properties are very important data in order to quantitatively understand the nanomachining process and to validate the FEM analysis results. Additionally, it is a very promising method for obtaining nanometer scale features on a large size specimen because it has a very wide working area and load range. The objective of this work is to suggest a maskless pattern fabrication technique of a Pyrex 7740 glass surface using the combination of machining by Nanoindenter XP and HF wet etching. The Pyrex 7740 glass (Pyrex glass) is one of the most representative silicatebased materials that are widely used in many fields of MEMS/NEMS and it is used as a bond material with silicon, a mold for nanoimprint lithography (NIL) and in medical devices, among other uses [14–16]. Sample line patterns were machined on a Pyrex 7740 glass surface by constant load scratch (CLS) of the Nanoindenter XP with a Berkovich diamond tip, and they were etched in HF solution to investigate chemical characteristics of the machined Pyrex 7740 glass surface. All morphological data of scratch traces were scanned using an atomic force microscope (AFM).
Fig. 1. Schematic diagram of (a) experimental procedure and (b) processing tip geometry.
2. Experimental procedure
Fig. 2. AFM scan of the diamond Berkovich indenter used in this study.
All scratch experiments were performed using a Nanoindenter XP (MTS, USA) with lateral force measurement option, and the experimental procedures are shown in Fig. 1(a). A Berkovch pyramid indenter (Fig. 1(b)) was used for scratch experiments because it produces plasticity at very low loads, it has good manufactured quality, and it minimized the influence of friction. A practical indenter has a non-zero tip radius, which is actually a sphere with a given radius. In addition, the wear of diamond tip can increase the radius of the tip. The tip radius not only influences the area-depth function of the indenter but also affects the deformation behavior of the materials during the scratch process. The AFM profiles of the diamond Berkovich indenter used in this study is shown in Fig. 2. Prior to a test, five indents were made in pure Al to clean the tip, and then two indents were made in a standard sample and fused silica to evaluate the condition of the tip. The sample used for all processes was a Pyrex glass 7740 (80.9 wt.% SiO2–12.7B2O3–2.3Al2O3– 4.0Na2O3–0.04K2O3–0.03Fe2O3) wafer of 500 lm thickness. The surface roughness Ra of the sample was less
than 5 nm as measured from 5 lm · 5 lm topographic AFM images. Specimens were initially cleaned with DI water in the ultrasonic bath. To fabricate the line patterns with the constant depth along the scratch direction, constant load scratch (CLS) was performed to vary the normal load and pattern pitch. For CLS conditions, a scratch length, scratch rate, and normal load were 300 lm, 10 lm/s, and (0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 mN), respectively. The orientation of the Berkovich indenter was aligned in a face forward direction in order to delaminate the overcoats of material during scratching. Fabricated line patterns on Pyrex 7740 glass surface were etched in 10 wt.% HF solution for 20 min. To avoid poor etching due to insufficient etchant provision, ultrasonic radiation was used. All specimens were scanned with a commercial AFM system, XE-100 (PSIA, South Korea). The CLS experiments and AFM observation were performed under atmospheric conditions at room temperature (20–21 C) with relative humidity ranging between 45% and 50%.
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3. Results and discussion 3.1. Ductile-regime nano/micro machining of borosilicate Eight grooves were machined on the surface by CLS with varying normal loads (Ln) of 0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, and 10 mN, as shown in Fig. 3. Crack-free grooves were observed because cutting depths were sufficiently small. The nano-deformation behavior of the Pyrex 7740 glass corresponded almost completely to the general elastoplastic deformation model, and sinking-in was observed It has been reported that for materials with a low value of E (Elastic modulus)/Y (Yield stress) such as some glasses and ceramics, the plastic zone is typically contained within the boundary of the circle of contact and the elastic deformations that accommodate the volume of the indentation are spread at a greater distance from the indenter [17]. Therefore, sinking-in is more likely to occur than piling-up. Fig. 4 shows the width and depth variation with increasing normal loads. It can be seen that the cutting traces with below 0.4 mN showed almost the same width and depth due to the bluntness of diamond tip. In the nano-scratch, morphology of cutting traces, such as width and depth, is determined by penetration depth of tip. Therefore, in order to achieve the nanometer scale resolution, very small penetration depth is needed. A problem is that a practical indenter has a non-zero tip radius, which is estimated as a sphere with a given radius. Therefore, when the cutting depth is too small, the deformation behavior of material is strongly affected by the elastic contact between the tip edge and the sur-
Fig. 4. Variation of the size and width of grooves with increasing normal load.
Fig. 5. The relationship between elastic recovery and normal load.
face. In Fig. 5, the relationship between elastic recovery and normal load was investigated from the cross profile topography and residual morphology profile graph in order to investigate the effect of tip radius on the elastic recovery of the material. It could be confirmed that the amount of elastic recovery increased with decreasing normal load due to the decrease of the cutting depth. A continuous stiffness measurement (CSM) test is another method to determine the minimum cutting depth, because indentation size effect (ISE), which is due to the elasticity of the tip edge, can be detected by variation of hardness as a function of a penetration depth [18]. 3.2. Groove pattern fabrication by constant load scratch (CLS)
Fig. 3. Grooves machined under various normal load conditions of 10, 5, 1, 0.5, 0.4, 0.3, 0.2, 0.1 mN (scratch velocity 10 lm/s, scratch length: 300 lm).
A constant load scratch (CLS) technique was used to make a pattern on a Pyrex 7740 glass surface and to characterize its friction behavior. One of the merits of pattern fabrication using Nanoindenter XP is that tribological characteristics such as friction coefficient and frictional force are measured simultaneously with machining. Fig. 6(a) shows cutting depth variation with increasing scratch distance; the normal load, scratch velocity, and scratch distance were 5 mN, 10 lm/s, and 300 lm, respectively. Under slow normal loading conditions, scratches create elastic–plastic deformation, which leads to a groove, accompanied by two lateral sinking-in pads.
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Fig. 6. Tribological characteristics of Pyrex 7740 glass surface with increasing scratch distance: (a) Penetration depths before and after elastic recovery; (b) Coefficient of friction; (c) Friction force (Normal load 5 mN, scratch velocity 10 lm/s).
Fig. 7. AFM images of groove patterns machined by the CLS under the different normal load and pitch: (a) Ln = 5 mN, Pl = 5 lm; (b) Ln = 5 mN, Pl = 1.5 lm; and (c) Ln = 1 mN, Pl = 600 nm.
These scratches are very similar along the scratch length, which indicate that surface deformation is consistent. Variations of friction force and friction coefficient as a function of scratch distance are shown in Fig. 6(b) and Fig. 6(c), respectively. Five line-patterns were machined for variation of pitch of lines (Fig. 7). As shown in Fig. 7(a) and (b), the specimens with pitch of 5 lm and 1.5 lm show sound results with respect to the uniformity of line width and pitch (Fig. 7(a), (b)). In the case of Fig. 7(c), both line pitch and normal load were reduced to 1 lm and 1 mN, respectively, in order to avoid the effect of the sinking-in of adjacent groove. However, the specimens with pitch of 1 lm show the somewhat irregular line pitch due to the limit of the position accuracy of x–y stage. In the scratch tests using the Nanoindenter XP system, once the indenter is in contact with the test surface, the x–y stage is moved. According to the manual of the Nanoindenter XP, position accuracy of the x–y stage is about 1.5 lm or better. 3.3. Modification of pattern structures by HF wet etching Fig. 8 shows a masking effect of the affected layer of scratched Pyrex 7740 glass surface. First, one groove was machined on the specimen surface by CLS with normal load of 5 mN and scratch velocity of 10 lm/s (Fig. 8(a)). Next, the machined sample was etched in 10 wt.% HF solutions for 20 min (Fig. 8(b)). After adequate washing and drying of the sample, changes in
Fig. 8. Morphological variation of the machined groove on Pyrex 7740 glass surface after 10 wt.% HF wet etching: (a) AFM image of grooves machined under the normal load condition of 5 mN; (b) AFM image of grooves after 10 wt.% HF etching for 20 min.
the processed part were observed by contact mode AFM. During experiments to investigate the chemical properties of the affected layer of the machined surface, an interesting result was found in that a convex structure was formed after the etching process. This implies that an affected layer of the machined surface has a masking effect in the wet etching by HF solution. After the etching process, the groove of 1 lm width and 60 nm depth were changed to convex structure of 1 lm width and 160 nm height. The aspect ratio of the convex structure was very low due to the isotropy of the amorphous Pyrex 7740 glass. However, it can be improved by anisotropic etching using a dry etching technology, such as the plasma etching process.
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Fig. 9. Shape variation of the machined groove on Pyrex 7740 glass surface after 10 wt.% HF wet etching: (a) AFM image of grooves machined under the normal load condition of 5 mN; (b) AFM image of grooves after 10 wt.% HF etching for 20 min.
were investigated with increasing scratch distance and the following results were obtained.
Fig. 10. Section profile change of grooves machined under the normal load of 5 mN before and after 10 wt.% HF wet etching.
The etch-mask effect of the mechanically affected layer on the scratched borosilicate surface has not yet been discovered. It is well known that the HF solution etches the SiO2 well, which is the main component of Pyrex 7740 glass. Therefore, it can be predicted that the chemical change of the scratched Pyrex 7740 glass may be occur due to the applied pressure or the friction between the diamond tip and the surface during nanoscratch process. In the next study, the affected layer of machined surface will be analyzed by using TEM section observation, Rutherford back scattering (RBS), and Auger electron spectroscopy (AES) to investigate the mechanism of the masking effect. Fig. 9 shows the AFM images of line pattern before and after etching process with 10 wt.% HF solution. The surface of the specimen was machined by CLS with normal load of 5 mN and scratch velocity of 10 lm/s (Fig. 9(a)), and it was etched in 10 wt.% HF solutions for 20 min (Fig. 9(b)). Section profile change of grooves before and after etching process is shown in Fig. 10. 4. Conclusions The present work investigated patterning of Pyrex 7740 glass surfaces using both nanoindenter and HF wet etching. Morphology of cutting traces, such as width and depth, were investigated with increasing applied normal loads (0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 mN). In addition, friction force, friction coefficient, and penetration depth variation before and after elastic recovery
(i) When the cutting depth was too small, the deformation behavior of material was strongly affected by the elastic contact between the tip edge and the surface. Cutting traces with 0.1, 0.2, 0.3, and 0.4 mN showed almost the same width and depth due to the bluntness of diamond tip. In addition, the amount of elastic recovery increased with decreasing normal load. (ii) Groove patterns with various pitches (5, 1.5, 1 lm) were fabricated by constant load scratch under the different normal loads (1, 5 mN) and scratch velocity of 10 lm/s. In these scratch conditions, it was found that a pattern pitch must be over 1 lm to achieve the sound pattern with uniform pitch. (iii) The nano-scratched Pyrex 7740 glass surface showed the sufficient etch-mask effect in HF solution. The 1-lm-wide and 160-nm-high convex structures were fabricated by etching the 1-lm-wide and 60-nm-deep grooves, which were machined on the Pyrex 7740 glass surface by constant load scratch (scratch velocity 10 lm/s, normal load 5 mN) in 10 wt.% HF solution for 20 min.
Acknowldgments This work has supported by Korea Research Foundation Grant (KRF-2003-041-D20042). The authors would like to express their deep gratitude to the KRF for its financial assistance. References [1] [2] [3] [4]
Dragoman D, Dragoman M. Prog Quant Electron 2001;25:229. Anderson H, van den Berg A. Sens Actuators B 2003;92:315. Li K, Shapiro Y, Li JCM. Acta mater 1998;46(15):5569. Shibata T, Fuji S, Makino E, Ikeda M. Jpan Prec Eng 1996; 18(2/3):130. [5] Tan J, Syoji K, Suzuki H, Kuriyagawa T. Jpan Prec Eng 1998;64(9):1345. [6] Campell PM, Snow ES. Proc SPIE-Intl Soc Optical Engr 1999;3975:943.
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S.W. Youn, C.G. Kang / Scripta Materialia 52 (2005) 117–122 Morita N. J of the JSGE 2001;45(6):275. Ashida K, Morita N, Yoshida Y. JSME Int J, SerC 2001;44(1):51. Miyake S, Kim JD. Jpn J Appl Phys 2002;41:L1116. Miyake S, Kim JD. Jpn J Appl Phys 2001;40:L1247. Youn SW, Kang CG. Scripta mater 2004;50:105. Chang LC, Brewer GJ, Wheeler BC. Biomaterials 2003;24:2863. Chou SY, Krauss PR, Renstrom P. J Appl Phys Lett 1995;67(21): 3114.
[14] Chou SY, Krauss PR. Microelectronic Engineering 1997;35: 237. [15] Gretillat MA, Paoletti F, Thiebaud P, Roth S, koudelka-Hap M, de Rooij NF. Sens Actuators A 1997;60:219. [16] Hulsenberg D. Microelectronics J 1997;28:419. [17] Fischer-Cripps Anthony C. Nanoindentation (Mech. Eng. Series). Berlin, Germany: Springer-Verlag; 2002. p. 71. [18] Lu CJ, Bogy DB. Int J Solids Structures 1995;32(12):1759.
Maskless pattern fabrication on Pyrex 7740 glass surface by using nano-scratch with HF wet etching S.W. Youn a
a,*
, C.G. Kang
b,1
Department of Precision and Mechanical Engineering, Pusan National University, San 30, JangJeon-Dong, Kumjung-Ku, P/Busan 609735, Korea b School of Mechanical Engineering, Pusan National University, P/Busan 609-735, Korea Received 19 March 2004; received in revised form 7 September 2004; accepted 17 September 2004
Abstract Line patterns were machined on borosilicate (Pyrex 7740 glass) surface using the combination of mechanical machining by Nanoindenter XP and HF wet etching, and a etch-mask effect of the affected layer of the nano-scratched Pyrex 7740 glass surface was investigated. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Lithography; Nano-scratch test; Silicate
1. Introduction The need for a low-priced patterning technique of hard-brittle materials such as silicon and glass has increased as a result of the development of NEMS/MEMS parts such as chromatography chips, lab-on-a-chip (LOC), system-on-chip (SOC), storage medias, opticallenses, filters, and others [1,2]. Hard-brittle materials such as silicon, glass and ceramics have two modes of materials removal, ductile and brittle. It is well known that a no-crack and fine machined surface can be achieved by a ductile-regime machining. Materials removal with plastic deformation, known as ductileregime machining, has been achieved by a very shallow cut. When the depth or the force of a cut exceeds some threshold value (i.e. critical depth or force) the material removal mode will change from ductile mode to brittle *
Corresponding author. Tel.: +82 51 510 1455; fax: +82 51 512 1722. E-mail addresses: [email protected] (S.W. Youn), cgkang@ pusan.ac.kr (C.G. Kang). 1 Tel.: +82 51 510 2335.
mode [3–5] The ductile-regime machining concept has been used in nanoprobes based on lithographic technology such as static/dynamic scratches using AFM and FFM [6–11]. A nano-scratch technique has been recognized as a potential maskless nanofabrication technique that can substitute for or supplement optical lithography because of its operational versatility, low costs for initial facilities and manufacture, simplicity of process, and material selectivity. There is an additional advantage that the size of structures can be varied by controlling the normal load applied to a single asperity tip that serves as the cutting tool. However, static/dynamic scratches themselves are not suitable for mass production because they are time-consuming methods and are not economical for commercial applications. One solution is to fabricate a mold that will be used for mass production processes such as nanoimprint, PDMS rheology casting, and others. It is important to fabricate a mold which has pattern structures over a large area of surface because the key advantages of the nanoimprint and the PDMS casting process are the ability to pattern nano/micro structures with a high-throughput and low cost [12–14].
1359-6462/$ - see front matter 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.09.016
118
S.W. Youn, C.G. Kang / Scripta Materialia 52 (2005) 117–122
From this point of view, we believe that the scratch experiment using a nanoindenter is one of the most promising possibilities for developing a nanomachining tool. Nanoindenters are often used for the measurement of mechanical properties such as hardness, elastic modulus, coefficient of friction, and lateral force of very thin films in any direction. Also, the amount of elastic recovery of materials can be measured from the cross profile topography and residual morphology profile graph. These mechanical properties are very important data in order to quantitatively understand the nanomachining process and to validate the FEM analysis results. Additionally, it is a very promising method for obtaining nanometer scale features on a large size specimen because it has a very wide working area and load range. The objective of this work is to suggest a maskless pattern fabrication technique of a Pyrex 7740 glass surface using the combination of machining by Nanoindenter XP and HF wet etching. The Pyrex 7740 glass (Pyrex glass) is one of the most representative silicatebased materials that are widely used in many fields of MEMS/NEMS and it is used as a bond material with silicon, a mold for nanoimprint lithography (NIL) and in medical devices, among other uses [14–16]. Sample line patterns were machined on a Pyrex 7740 glass surface by constant load scratch (CLS) of the Nanoindenter XP with a Berkovich diamond tip, and they were etched in HF solution to investigate chemical characteristics of the machined Pyrex 7740 glass surface. All morphological data of scratch traces were scanned using an atomic force microscope (AFM).
Fig. 1. Schematic diagram of (a) experimental procedure and (b) processing tip geometry.
2. Experimental procedure
Fig. 2. AFM scan of the diamond Berkovich indenter used in this study.
All scratch experiments were performed using a Nanoindenter XP (MTS, USA) with lateral force measurement option, and the experimental procedures are shown in Fig. 1(a). A Berkovch pyramid indenter (Fig. 1(b)) was used for scratch experiments because it produces plasticity at very low loads, it has good manufactured quality, and it minimized the influence of friction. A practical indenter has a non-zero tip radius, which is actually a sphere with a given radius. In addition, the wear of diamond tip can increase the radius of the tip. The tip radius not only influences the area-depth function of the indenter but also affects the deformation behavior of the materials during the scratch process. The AFM profiles of the diamond Berkovich indenter used in this study is shown in Fig. 2. Prior to a test, five indents were made in pure Al to clean the tip, and then two indents were made in a standard sample and fused silica to evaluate the condition of the tip. The sample used for all processes was a Pyrex glass 7740 (80.9 wt.% SiO2–12.7B2O3–2.3Al2O3– 4.0Na2O3–0.04K2O3–0.03Fe2O3) wafer of 500 lm thickness. The surface roughness Ra of the sample was less
than 5 nm as measured from 5 lm · 5 lm topographic AFM images. Specimens were initially cleaned with DI water in the ultrasonic bath. To fabricate the line patterns with the constant depth along the scratch direction, constant load scratch (CLS) was performed to vary the normal load and pattern pitch. For CLS conditions, a scratch length, scratch rate, and normal load were 300 lm, 10 lm/s, and (0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 mN), respectively. The orientation of the Berkovich indenter was aligned in a face forward direction in order to delaminate the overcoats of material during scratching. Fabricated line patterns on Pyrex 7740 glass surface were etched in 10 wt.% HF solution for 20 min. To avoid poor etching due to insufficient etchant provision, ultrasonic radiation was used. All specimens were scanned with a commercial AFM system, XE-100 (PSIA, South Korea). The CLS experiments and AFM observation were performed under atmospheric conditions at room temperature (20–21 C) with relative humidity ranging between 45% and 50%.
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3. Results and discussion 3.1. Ductile-regime nano/micro machining of borosilicate Eight grooves were machined on the surface by CLS with varying normal loads (Ln) of 0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, and 10 mN, as shown in Fig. 3. Crack-free grooves were observed because cutting depths were sufficiently small. The nano-deformation behavior of the Pyrex 7740 glass corresponded almost completely to the general elastoplastic deformation model, and sinking-in was observed It has been reported that for materials with a low value of E (Elastic modulus)/Y (Yield stress) such as some glasses and ceramics, the plastic zone is typically contained within the boundary of the circle of contact and the elastic deformations that accommodate the volume of the indentation are spread at a greater distance from the indenter [17]. Therefore, sinking-in is more likely to occur than piling-up. Fig. 4 shows the width and depth variation with increasing normal loads. It can be seen that the cutting traces with below 0.4 mN showed almost the same width and depth due to the bluntness of diamond tip. In the nano-scratch, morphology of cutting traces, such as width and depth, is determined by penetration depth of tip. Therefore, in order to achieve the nanometer scale resolution, very small penetration depth is needed. A problem is that a practical indenter has a non-zero tip radius, which is estimated as a sphere with a given radius. Therefore, when the cutting depth is too small, the deformation behavior of material is strongly affected by the elastic contact between the tip edge and the sur-
Fig. 4. Variation of the size and width of grooves with increasing normal load.
Fig. 5. The relationship between elastic recovery and normal load.
face. In Fig. 5, the relationship between elastic recovery and normal load was investigated from the cross profile topography and residual morphology profile graph in order to investigate the effect of tip radius on the elastic recovery of the material. It could be confirmed that the amount of elastic recovery increased with decreasing normal load due to the decrease of the cutting depth. A continuous stiffness measurement (CSM) test is another method to determine the minimum cutting depth, because indentation size effect (ISE), which is due to the elasticity of the tip edge, can be detected by variation of hardness as a function of a penetration depth [18]. 3.2. Groove pattern fabrication by constant load scratch (CLS)
Fig. 3. Grooves machined under various normal load conditions of 10, 5, 1, 0.5, 0.4, 0.3, 0.2, 0.1 mN (scratch velocity 10 lm/s, scratch length: 300 lm).
A constant load scratch (CLS) technique was used to make a pattern on a Pyrex 7740 glass surface and to characterize its friction behavior. One of the merits of pattern fabrication using Nanoindenter XP is that tribological characteristics such as friction coefficient and frictional force are measured simultaneously with machining. Fig. 6(a) shows cutting depth variation with increasing scratch distance; the normal load, scratch velocity, and scratch distance were 5 mN, 10 lm/s, and 300 lm, respectively. Under slow normal loading conditions, scratches create elastic–plastic deformation, which leads to a groove, accompanied by two lateral sinking-in pads.
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S.W. Youn, C.G. Kang / Scripta Materialia 52 (2005) 117–122
Fig. 6. Tribological characteristics of Pyrex 7740 glass surface with increasing scratch distance: (a) Penetration depths before and after elastic recovery; (b) Coefficient of friction; (c) Friction force (Normal load 5 mN, scratch velocity 10 lm/s).
Fig. 7. AFM images of groove patterns machined by the CLS under the different normal load and pitch: (a) Ln = 5 mN, Pl = 5 lm; (b) Ln = 5 mN, Pl = 1.5 lm; and (c) Ln = 1 mN, Pl = 600 nm.
These scratches are very similar along the scratch length, which indicate that surface deformation is consistent. Variations of friction force and friction coefficient as a function of scratch distance are shown in Fig. 6(b) and Fig. 6(c), respectively. Five line-patterns were machined for variation of pitch of lines (Fig. 7). As shown in Fig. 7(a) and (b), the specimens with pitch of 5 lm and 1.5 lm show sound results with respect to the uniformity of line width and pitch (Fig. 7(a), (b)). In the case of Fig. 7(c), both line pitch and normal load were reduced to 1 lm and 1 mN, respectively, in order to avoid the effect of the sinking-in of adjacent groove. However, the specimens with pitch of 1 lm show the somewhat irregular line pitch due to the limit of the position accuracy of x–y stage. In the scratch tests using the Nanoindenter XP system, once the indenter is in contact with the test surface, the x–y stage is moved. According to the manual of the Nanoindenter XP, position accuracy of the x–y stage is about 1.5 lm or better. 3.3. Modification of pattern structures by HF wet etching Fig. 8 shows a masking effect of the affected layer of scratched Pyrex 7740 glass surface. First, one groove was machined on the specimen surface by CLS with normal load of 5 mN and scratch velocity of 10 lm/s (Fig. 8(a)). Next, the machined sample was etched in 10 wt.% HF solutions for 20 min (Fig. 8(b)). After adequate washing and drying of the sample, changes in
Fig. 8. Morphological variation of the machined groove on Pyrex 7740 glass surface after 10 wt.% HF wet etching: (a) AFM image of grooves machined under the normal load condition of 5 mN; (b) AFM image of grooves after 10 wt.% HF etching for 20 min.
the processed part were observed by contact mode AFM. During experiments to investigate the chemical properties of the affected layer of the machined surface, an interesting result was found in that a convex structure was formed after the etching process. This implies that an affected layer of the machined surface has a masking effect in the wet etching by HF solution. After the etching process, the groove of 1 lm width and 60 nm depth were changed to convex structure of 1 lm width and 160 nm height. The aspect ratio of the convex structure was very low due to the isotropy of the amorphous Pyrex 7740 glass. However, it can be improved by anisotropic etching using a dry etching technology, such as the plasma etching process.
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Fig. 9. Shape variation of the machined groove on Pyrex 7740 glass surface after 10 wt.% HF wet etching: (a) AFM image of grooves machined under the normal load condition of 5 mN; (b) AFM image of grooves after 10 wt.% HF etching for 20 min.
were investigated with increasing scratch distance and the following results were obtained.
Fig. 10. Section profile change of grooves machined under the normal load of 5 mN before and after 10 wt.% HF wet etching.
The etch-mask effect of the mechanically affected layer on the scratched borosilicate surface has not yet been discovered. It is well known that the HF solution etches the SiO2 well, which is the main component of Pyrex 7740 glass. Therefore, it can be predicted that the chemical change of the scratched Pyrex 7740 glass may be occur due to the applied pressure or the friction between the diamond tip and the surface during nanoscratch process. In the next study, the affected layer of machined surface will be analyzed by using TEM section observation, Rutherford back scattering (RBS), and Auger electron spectroscopy (AES) to investigate the mechanism of the masking effect. Fig. 9 shows the AFM images of line pattern before and after etching process with 10 wt.% HF solution. The surface of the specimen was machined by CLS with normal load of 5 mN and scratch velocity of 10 lm/s (Fig. 9(a)), and it was etched in 10 wt.% HF solutions for 20 min (Fig. 9(b)). Section profile change of grooves before and after etching process is shown in Fig. 10. 4. Conclusions The present work investigated patterning of Pyrex 7740 glass surfaces using both nanoindenter and HF wet etching. Morphology of cutting traces, such as width and depth, were investigated with increasing applied normal loads (0.1, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 mN). In addition, friction force, friction coefficient, and penetration depth variation before and after elastic recovery
(i) When the cutting depth was too small, the deformation behavior of material was strongly affected by the elastic contact between the tip edge and the surface. Cutting traces with 0.1, 0.2, 0.3, and 0.4 mN showed almost the same width and depth due to the bluntness of diamond tip. In addition, the amount of elastic recovery increased with decreasing normal load. (ii) Groove patterns with various pitches (5, 1.5, 1 lm) were fabricated by constant load scratch under the different normal loads (1, 5 mN) and scratch velocity of 10 lm/s. In these scratch conditions, it was found that a pattern pitch must be over 1 lm to achieve the sound pattern with uniform pitch. (iii) The nano-scratched Pyrex 7740 glass surface showed the sufficient etch-mask effect in HF solution. The 1-lm-wide and 160-nm-high convex structures were fabricated by etching the 1-lm-wide and 60-nm-deep grooves, which were machined on the Pyrex 7740 glass surface by constant load scratch (scratch velocity 10 lm/s, normal load 5 mN) in 10 wt.% HF solution for 20 min.
Acknowldgments This work has supported by Korea Research Foundation Grant (KRF-2003-041-D20042). The authors would like to express their deep gratitude to the KRF for its financial assistance. References [1] [2] [3] [4]
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