- Email: [email protected]
Achieving stable removal rate in polishing with small tools
Precision Engineering xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Achieving stable removal rate in polishing with small tools Urara Satakea,∗, Toshiyuki Enomotoa, Teppei Miyagawaa, Takuya Ohsumia, Hidenori Nakagawab, Katsuhiro Funabashib a b
Department of Mechanical Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japan Production Engineering Research Laboratory, Production Engineering, Headquarters, Canon Inc., 23-10 Kiyohara-kogyodanchi, Utsunomiya, Tochigi, 321-3298, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Polishing Removal rate Glass lenses
The demand for cameras with high image quality has led to their increased use by consumers and in industrial applications such as broadcasting, on-vehicle, security, and medicine. Glass lenses as key devices in cameras are finished with small polishing tools. However, the existing polishing technologies have serious problems, including an unstable material removal rate over time. To investigate the variation mechanism of the material removal rate, glass-polishing experiments were conducted. Based on our findings, we proposed a vibrationassisted polishing method using newly developed polishing pads containing titanium dioxide particles. Our experiments revealed that the proposed polishing method significantly improved the stability of the material removal rate.
1. Introduction Recently, the camera technology market has rapidly expanded in the consumer and industrial sectors. The expanding range of applications is accompanied by increasing demands for image quality. Glass lenses are a key driver of image quality; they must therefore be finished using processes that produce extremely precise shapes. Glass optical elements, particularly aspherical lenses, are polished using small polishing tools [1–3]. Polishing pads attached to the end of rotating polishing tools travel over the surface of the glass workpiece to shape the lens. A precise amount of material must be removed from each location on the workpiece. A sequence of polishing parameters are set beforehand on the basis of the Preston equation [4], r = kPv, which indicates that the material removal rate r is proportional to the applied polishing pressure P and to the velocity v of the tool relative to the workpiece. The Preston equation also includes a Preston coefficient k, which mainly depends on the number of active abrasive particles on the polishing pad surface. The surface conditions of polishing pads change over the course of the polishing process, leading to changes in the Preston coefficient over time [5–18]. This change destabilizes the material removal rate as the polishing time accumulates, thereby deteriorating the precision of the finished shape of workpiece. In fabrication processes, the surfaces of polishing pads are frequently conditioned to stabilize the surface conditions of pads, leading to increase in production costs. In this study, the relation between the material removal rate and the
∗
surface conditions of polishing pads is studied through glass-polishing experiments. Based on the findings, a vibration-assisted polishing method that uses a newly developed polishing pad with titanium dioxide (TiO2) particles is proposed. This polishing method stabilizes the removal rate, as demonstrated via comparisons with the performance of the conventional polishing method using a commercially available polishing pad. 2. Glass-polishing characteristics 2.1. Glass-polishing experiments by a conventional polishing method Polishing experiments were conducted on the panes of synthetic quartz glass using a small polishing tool, as illustrated in Fig. 1; Table 1 lists the polishing conditions. The polishing tool comprised a pipeshaped shaft and an annular polishing pad (outer diameter = 6 mm and inner diameter = 2 mm). Commercial polyurethane foam polishing pads with fillers composed of cerium dioxide particles, which are commonly used in glass polishing, were tested. Before the series of polishing process, the surface of the polishing pad was conditioned with a #1000 diamond disk. The rotating polishing tool was held perpendicular to the workpiece surface and scanned over the surface at a speed of 10 mm/s. A spring holding the polishing tool maintained a constant polishing pressure of 195 kPa. Slurry was supplied through the center of the shaft and emerged from a hole in the center of the pad. The material removal rate was calculated from the removal profiles,
Corresponding author. E-mail address: [email protected] (U. Satake).
https://doi.org/10.1016/j.precisioneng.2018.09.012 Received 9 May 2018; Received in revised form 15 August 2018; Accepted 5 September 2018 0141-6359/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Satake, U., Precision Engineering, https://doi.org/10.1016/j.precisioneng.2018.09.012
Precision Engineering xxx (xxxx) xxx–xxx
U. Satake et al.
Polishing pad
Area of burr mm2
Slurry
Shaft
2.0 1.5 1.0 0.5 0 0
Workpiece Fig. 1. Experimental setup for polishing tests.
Polishing pad
Rotational speed of tool Scanning speed of tool Polishing pressure Slurry Slurry supply rate
2.2. Mechanism of low removal rate at the beginning of polishing process
Removal rate mm3/h
The trend of low removal rate at the beginning of polishing process, as previously described, is commonly observed in fabrication processes. This part of the process is referred to as “dummy polishing”, and it considerably increases the production cost. The material removal rate at the beginning of polishing process is well known to be deteriorated
9 8
Variation rate 10%
7 Max. variation rate 28%
5
0
1
2
8
9
3 4 5 6 7 Polishing session
Pore
Plateau
4 3 0
3 4 5 6 7 Polishing session
by burrs on the surface of polishing pad generated during the surface conditioning process [19]. Fig. 3 shows the burr area on polishing pad surface before each polishing session, as determined via image analysis of laser scanning microscope (VK-X260; Keyence Corp.) images. We evaluated the area where the surface was higher than the most frequent height in surface height map of the entire polishing pad by more than 15 μm as the burr area. The large area on pad surface was covered with the burrs before first polishing session due to the surface conditioning with a diamond disk, and then almost all the burrs immediately disappeared during first session. This result suggests that burr generation was not responsible for the low removal rate during the first half of the polishing process (Fig. 2). We estimated the number of abrasive particles on the polishing pad surface to investigate the reason for low material removal rate at the beginning of the process. The surface of the polishing pad comprised pores ranging from 30 to 500 μm (average: 160 μm) in diameter and plateaus between each pore (Fig. 4). The abrasive particles held on the plateau area are able to remove material from the workpiece, that is, they can act as the active abrasive particles. We determined the plateau area with the abrasive particles using scanning electron microscopy (SEM; TM3000; Hitachi HighTechnologies Corp.) images of the polishing pad surface after each polishing session. Fig. 5(a) shows the SEM image of the pad surface after the final polishing session. White spots in the image represent remaining material, which was confirmed to comprise only abrasive particles by energy-dispersive X-ray spectrometry (EDX; S-3000; Hitachi High-Technologies Corp.) analysis. The area with higher brightness on the plateau in the images was evaluated as the plateau area with the abrasive particles. Fig. 5(b) shows the ratio of the plateau area with the abrasive particles to the pad surface area after each polishing session. The plateau area with the abrasive particles was small during the first half of the polishing process, particularly, during the first and second sessions (Fig. 5(b)), corresponding to the result of the material removal rate (Fig. 2). To investigate the small number of the active abrasive particles at the beginning of the polishing process, we estimated the number of abrasive particles on the polishing pad surface existing before each
Synthetic quartz glass Diameter: 100 mm Polyurethane foam-polishing pad with cerium dioxide particles Outer diameter: 6 mm; inner diameter: 2 mm 10,000 rpm 10 mm/s 195 kPa Ceria (mean diameter: 1 μm), 1 wt% 500 mL/min
which were measured using an optical profiler (NewView 200; Zygo Corp.). The 10-min polishing session was continuously repeated eight times in the series of polishing process. To remove glass chips generated during polishing session, the surface of polishing pad was cleaned by a nylon brush before the next session. Fig. 2 shows a plot of the changes in the material removal rate vs. polishing session. The material removal rate dramatically changed as polishing progressed. The maximum variation in the removal rate was 28%; however, manufacturers require < 10% variance (grey shaded area in Fig. 2). The low removal rate during the first half of the polishing process, particularly, during the first and second sessions and the sudden decrease in the removal rate during the final session significantly deteriorated stability of the material removal rate.
6
2
Fig. 3. Changes in the burr area on polishing pad surface before polishing session.
Table 1 Glass polishing conditions. Workpiece
1
8
9 200
Fig. 2. Changes in material removal rate with accumulated polishing sessions using a commercially available polishing pad.
Fig. 4. Optical microscope image of the surface of a commercial polishing pad. 2
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Abrasive particles Polishing pad
Abrasive particle
1 mm Polishing pad
(a) Scanning electron microscopy image of the surface of a commercial polishing pad after the final polishing session
Area ratio of abrasive %
Workpiece
14 12 10 8 6 4 2 0
Pore area
Plateau area
Active abrasive particle
(a) Model of behavior of an abrasive particle on polishing pad surface
At the beginning of polishing process
0
1
2
3 4 5 6 7 Polishing session
8
9 After polishing process progressed
(b) Changes in the ratio of the plateau area with the abrasive particles after polishing session using a commercial polishing pad
Area ratio of abrasive %
Fig. 5. Surface conditions of a commercial polishing pad after polishing session.
14 12 10 8 6 4 2 0
(b) Mechanism of the low material removal rate at the beginning of polishing process Fig. 7. Mechanism of material removal in polishing process.
Pore
mechanism of the low material removal rate at the beginning of polishing process. Slurry supplied on polishing pad surface has difficulties in penetrating into the contact region between the plateau and the workpiece surface because of the considerably small gap between the surfaces; hence, most of the abrasive particles cannot be held directly on the plateau area. As illustrated in Fig. 7(a), they are held on the pore areas first. Then, some of them are transported from the pore area to the contact region. Finally, they are held on the plateau area and act as the active abrasive particles. At the beginning of polishing process, as illustrated in Fig. 7(b), only a few abrasive particles exist on the pore area. A small number of the abrasive particles, thus, can be transported to the contact region between the plateau and the workpiece surface, resulting in low material removal rate. As the polishing process progresses, the pore area becomes filled with abrasive particles, and then a large number of the particles can be transported to the contact region, leading to high material removal rate.
Plateau
0
1
2
3 4 5 6 7 Polishing session
8
9
Fig. 6. Changes in the ratio of the plateau area and the pore area with the abrasive particles before polishing session using a commercial polishing pad.
polishing session. Fig. 6 plots the ratio of the plateau area (green dots) and the pore area (red dots) with the abrasive particles to the pad surface area before each polishing session. As revealed by a comparison of the results for the plateau area after session (Fig. 5(b)) and before session (green dots in Fig. 6), the abrasive particles held on the plateau area during polishing session were almost completely removed by brushing process before the next session. Thus, there is little difference in the initial number of the abrasive particles on the plateau area between any eight polishing sessions (green dots in Fig. 6). On the other hand, the abrasive particles were accumulated on the pore area as the polishing process progressed (red dots in Fig. 6). The number of abrasive particles on the pore area existing before session was small at the beginning of polishing process and increased gradually, as with the number of the active abrasive particles (Fig. 5(b)). Based on the aforementioned results, we propose the following
2.3. Mechanism of sudden decrease in removal rate during polishing process The material removal rate dropped by 18% during the final session of the polishing process (Fig. 2). A large number of studies have reported that surface of polishing pad become worn and glazed over time during polishing process, which leads to decrease in material removal rate [5,7]. Fig. 8 shows a plot of the glazed area on the polishing pad surface after each polishing session. The glazed area was measured in terms of the number of pixels that were glazed, that is, reflected larger quantity of laser beam in images of the polishing pad surface measured by a laser scanning microscope. The dramatic increase in the glazed 3
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Removal rate mm3/h
Index of glazed area
U. Satake et al.
6000 5000 4000 3000 2000 0
1
2 3 4 5 6 7 Polishing session
8
9
Fig. 8. Changes in the glazed area on polishing pad surface after polishing session.
9 8 7 Max. variation rate 13%
6 5 4 3 0
0
1
2
3 4 5 6 7 Polishing session
8
9
Fig. 12. Changes in material removal rate with accumulated polishing sessions using a newly developed polishing pad.
Area ratio of abrasive %
Pore Plateau
100 200 m Fig. 9. Optical microscope image of the surface of a developed polishing pad.
14 12 10 8 6 4 2 0 0
1
2
3 4 5 6 7 Polishing session
8
9
Fig. 13. Changes in the ratio of the plateau area with the abrasive particles after polishing session using a newly developed polishing pad.
10
4.0 μm SRz; 0.3 μm SRa
5.3 μm SRz; 0.3 μm SRa
(a) Commercial pad
(b) Newly developed pad
Area ratio of abrasive %
10
Fig. 10. Plateau area of polishing pad surfaces after conditioning with a diamond disk.
Droplet
1 mm
Contact angle: 17°
(a) Commercial pad
(b) Newly developed pad
Pore
Plateau
0
1
2
3 4 5 6 7 Polishing session
8
9
Fig. 14. Changes in the ratio of the plateau area and the pore area with the abrasive particles before polishing session using a newly developed polishing pad.
1 mm
Contact angle: 96°
14 12 10 8 6 4 2 0
The glazed pad surface was indicated to have negligible effect on removal rate also from the result during the first half of the polishing process, where both the removal rate (Fig. 2) and the glazed area on the pad surface (Fig. 8) gradually increased. We performed EDX analysis of the polishing pad surface after the final session to investigate the sudden decrease in removal rate. The result of the EDX analysis and laser scanning microscope observations of the pad surface revealed that huge workpiece debris (height of
Fig. 11. Wettability of the polishing pad surfaces.
area was not observed during the final polishing session, which indicates that the sudden decrease in the removal rate during the final session (Fig. 2) was not caused by the glazed surface of polishing pad. 4
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Removal rate mm3/h
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9 8
unlike in the case of the commercial pads (Fig. 5(b)). These results indicate that the abrasive particles supplied on the newly developed pad surface were held directly on the plateau area. The 13% variation in the material removal rate (Fig. 12) was, however, still higher than the industry demand of 10%. As shown in Fig. 12, the stability of the removal rate was deteriorated by the decrease in the removal rate during the third polishing session. The observations of the polishing pad surface revealed that it was attributed to the adhesion of tiny workpiece fragments to the pad surface. To enhance the self-cleaning effect of the developed polishing pad surface by promoting the penetration of slurry into the contact region between the plateau on pad surface and the workpiece surface, we applied vibration at a 0.2-Hz frequency and a 19.5-kPa amplitude to the polishing tool in the direction normal to the workpiece surface. The vibration was generated by varying the length of the spring that maintains the polishing pressure. To minimize the direct influence of the varying polishing pressure on the removal rate, the polishing tool was scanned considering the phase difference of the vibration. Fig. 15 shows that the maximum variation in the removal rate was reduced to 8% by a vibration-assisted polishing method with polishing pads containing TiO2 particles. The surface quality of the polished workpiece measured by an optical profiler was equivalent to that obtained by a conventional polishing method with commercial pads at 4nm P-V and 0.18-nm SRa in an area of 0.26 × 0.30 mm.
Max. variation rate 8%
7 6 5 4 3 0
0
1
2
3 4 5 6 7 Polishing session
8
9
Fig. 15. Changes in material removal rate in vibration-assisted polishing using a newly developed polishing pad.
∼10 μm) were adhered to the pad surface. The polishing tool was confirmed to be pushed up by ∼10 μm during the final polishing session by the position data of the polishing tool along the axis normal to the workpiece surface measured using a laser displacement meter. This result indicates that the adhesion of huge workpiece debris to the pad surface can drastically decrease material removal rate by creating a considerably large gap between the pad surface and the workpiece surface.
4. Conclusions To suppress the variation in the material removal rate with the accumulated polishing time in polishing with small tools, a vibrationassisted polishing method using polishing pads with TiO2 particles was developed. Our experimental results showed that the proposed method significantly improved the stability of the material removal rate by promoting the penetration of slurry into the contact region between the polishing pad surface and the workpiece surface due to the high hydrophilicity of the TiO2 particles and the vibration of the polishing tool.
3. Newly developed polishing method To promote the penetration of slurry into the contact region between the plateau on polishing pad surface and the workpiece surface, we developed a new polishing pad with TiO2 particles as fillers. TiO2 particles are highly hydrophilic, which should enhance the wettability of the pad surface. The penetration of more slurry into the contact region enables the abrasive particles to be held directly on the plateau area and prevents strong adhesion of workpiece debris to the pad surface by self-cleaning effect [20]. Rutile TiO2 spherical particles (TITANIX JR-603; TAYCA Corp.; mean diameter = 0.3 μm, TiO2 > 90%) were contained at 10 wt% in a polyurethane-foam-type polishing pad. Fig. 9 shows the surface of the developed polishing pad with exposed pores with diameters ranging from 30 to 800 μm (average: 170 μm). Observations of the plateau on the pad surface indicate that the developed polishing pads have slightly different surface textures and approximately same surface roughness compared with those of the commercial polishing pads (Fig. 10). The type-C rubber hardness of the developed pad measured using a durometer (GD-703 N; TECLOCK Corp.) is identical to that of the commercial pad at 55. Water-contact-angle measurements performed using a contact angle meter (DM-300; Kyowa Interface Science Co., Ltd.) confirms that the developed pad surface is considerably more hydrophilic than the commercial pad surface (Fig. 11). As shown in Fig. 12, the developed polishing pads with TiO2 particles solved the problems associated with low removal rate at the beginning of polishing process and sudden decrease in removal rate induced by adhesion of huge workpiece debris. Consequently, the maximum variation in the removal rate was suppressed to 13%. Fig. 13 shows the plateau area with the abrasive particles after each polishing session and Fig. 14 shows the plateau area (green dots) and the pore area (red dots) with the abrasive particles before each session. The number of the abrasive particles on the pore area before session was small at the beginning of polishing process (red dots in Fig. 14) as in the case of using the commercial pads (red dots in Fig. 6). On the other hand, many abrasive particles were observed on the plateau area after session from the beginning of the polishing process (Fig. 13)
References [1] Jones RA. Grinding and polishing with small tools under computer control. Opt Eng 1979;18/4:390–3. [2] Komanduri R, Lucca DA, Tani Y. Technological advances in fine abrasive processes. Ann CIRP 1997;46/2:545–96. [3] Suzuki H, Moriwaki T, Okino T, Ando Y. Development of ultrasonic vibration assisted polishing machine for micro aspherical die and mold. Ann CIRP 2006;55/ 1:385–8. [4] Preston FW. The theory and design of plate glass polishing machines. J Soc Glass Technol 1927;11:214–56. [5] McGrath J, Davis C. Polishing pad surface characteristics in chemical mechanical planarization. J Mater Process Technol 2004;153–154:666–73. [6] Wang C, Sherman P, Chandra A, Dornfeld D. Pad surface roughness and slurry particle size distribution effects on material removal rate in chemical mechanical planarization. Ann CIRP 2005;54/1:309–12. [7] Hooper BJ, Byrne G, Galligan S. Pad conditioning in chemical mechanical polishing. J Mater Process Technol 2002;123:107–13. [8] Uneda M, Maeda Y, Ishikawa K, Shibuya K, Nakamura Y, Ichikawa K, Doi T. Effect of pad surface asperity on removal rate in chemical mechanical polishing. Adv Mater Res 2012;497:256–63. [9] Kim S, Saka N, Chun J. The effect of pad-asperity curvature on material removal rate in chemical-mechanical polishing. Proced CIRP 2014;14:42–7. [10] Uneda M, Maeda Y, Shibuya K, Nakamura Y, Ichikawa D, Ishikawa K. Development of system for evaluation of polishing pad surface asperity: effects of pad surface asperity on removal rate. J Jpn Soc Grind Eng 2013;57/6:381–6. [11] Fukuda A, Mitarai M. Method for slurry flow visualization in polishing pad asperities. J Jpn Soc Precis Eng 2017;83/2:173–9. [12] Fukuda A. Microscale circulation flow of slurry in polishing pad asperities: results of flow visualization of slurry by using enlarged pad model. J Jpn Soc Precis Eng 2018;84/2:182–7. [13] Guiot A, Pattofatto S, Tournier C, Mathieu L. Modeling of a polishing tool to simulate material removal. Adv Mater Res 2011;223:784–93. [14] Byrne G, Mullany B, Young P. The effect of pad wear on the chemical mechanical polishing of silicon wafers. Ann CIRP 1999;48/1:143–6. [15] Jeong HD, Park KH, Cho KK. CMP pad break-in time reduction in silicon wafer polishing. Ann CIRP 2007;56/1:357–60. [16] Coppeta J, Rogers C, Racz L, Philipossian A, Kaufman FB. Investigating slurry
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2013. 52/12R: 126503-1-126503-6. [19] Yan JC, Choi JH, Hwang T, Lee CG, Kim T. Effects of diamond size of CMP conditioner on wafer removal rates and defects for solid (Non-Porous) CMP pad with micro-holes. Int J Mach Tool Manufact 2010;50:860–8. [20] Latthe SS, Liu S, Terashima C, Nakata K, Fujishima A. Transparent, adherent, and photocatalytic SiO2-TiO2 coatings on polycarbonate for self-cleaning applications. Coatings 2014;4:497–507.
transport beneath a wafer during chemical mechanical polishing processes. Electrochem Soc 2000;147/5:1903–9. [17] Park KH, Kim HJ, Chang OM, Jeong HD. Effects of pad properties on material removal in chemical mechanical polishing. J Mater Process Technol 2007;187–188:73–6. [18] Isobe A, Akaji M, Kurokawa S. Proposal of new polishing mechanism based on feret's diameter of contact area between polishing pad and wafer. Jpn Soc Appl Phys
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Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Achieving stable removal rate in polishing with small tools Urara Satakea,∗, Toshiyuki Enomotoa, Teppei Miyagawaa, Takuya Ohsumia, Hidenori Nakagawab, Katsuhiro Funabashib a b
Department of Mechanical Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japan Production Engineering Research Laboratory, Production Engineering, Headquarters, Canon Inc., 23-10 Kiyohara-kogyodanchi, Utsunomiya, Tochigi, 321-3298, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Polishing Removal rate Glass lenses
The demand for cameras with high image quality has led to their increased use by consumers and in industrial applications such as broadcasting, on-vehicle, security, and medicine. Glass lenses as key devices in cameras are finished with small polishing tools. However, the existing polishing technologies have serious problems, including an unstable material removal rate over time. To investigate the variation mechanism of the material removal rate, glass-polishing experiments were conducted. Based on our findings, we proposed a vibrationassisted polishing method using newly developed polishing pads containing titanium dioxide particles. Our experiments revealed that the proposed polishing method significantly improved the stability of the material removal rate.
1. Introduction Recently, the camera technology market has rapidly expanded in the consumer and industrial sectors. The expanding range of applications is accompanied by increasing demands for image quality. Glass lenses are a key driver of image quality; they must therefore be finished using processes that produce extremely precise shapes. Glass optical elements, particularly aspherical lenses, are polished using small polishing tools [1–3]. Polishing pads attached to the end of rotating polishing tools travel over the surface of the glass workpiece to shape the lens. A precise amount of material must be removed from each location on the workpiece. A sequence of polishing parameters are set beforehand on the basis of the Preston equation [4], r = kPv, which indicates that the material removal rate r is proportional to the applied polishing pressure P and to the velocity v of the tool relative to the workpiece. The Preston equation also includes a Preston coefficient k, which mainly depends on the number of active abrasive particles on the polishing pad surface. The surface conditions of polishing pads change over the course of the polishing process, leading to changes in the Preston coefficient over time [5–18]. This change destabilizes the material removal rate as the polishing time accumulates, thereby deteriorating the precision of the finished shape of workpiece. In fabrication processes, the surfaces of polishing pads are frequently conditioned to stabilize the surface conditions of pads, leading to increase in production costs. In this study, the relation between the material removal rate and the
∗
surface conditions of polishing pads is studied through glass-polishing experiments. Based on the findings, a vibration-assisted polishing method that uses a newly developed polishing pad with titanium dioxide (TiO2) particles is proposed. This polishing method stabilizes the removal rate, as demonstrated via comparisons with the performance of the conventional polishing method using a commercially available polishing pad. 2. Glass-polishing characteristics 2.1. Glass-polishing experiments by a conventional polishing method Polishing experiments were conducted on the panes of synthetic quartz glass using a small polishing tool, as illustrated in Fig. 1; Table 1 lists the polishing conditions. The polishing tool comprised a pipeshaped shaft and an annular polishing pad (outer diameter = 6 mm and inner diameter = 2 mm). Commercial polyurethane foam polishing pads with fillers composed of cerium dioxide particles, which are commonly used in glass polishing, were tested. Before the series of polishing process, the surface of the polishing pad was conditioned with a #1000 diamond disk. The rotating polishing tool was held perpendicular to the workpiece surface and scanned over the surface at a speed of 10 mm/s. A spring holding the polishing tool maintained a constant polishing pressure of 195 kPa. Slurry was supplied through the center of the shaft and emerged from a hole in the center of the pad. The material removal rate was calculated from the removal profiles,
Corresponding author. E-mail address: [email protected] (U. Satake).
https://doi.org/10.1016/j.precisioneng.2018.09.012 Received 9 May 2018; Received in revised form 15 August 2018; Accepted 5 September 2018 0141-6359/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Satake, U., Precision Engineering, https://doi.org/10.1016/j.precisioneng.2018.09.012
Precision Engineering xxx (xxxx) xxx–xxx
U. Satake et al.
Polishing pad
Area of burr mm2
Slurry
Shaft
2.0 1.5 1.0 0.5 0 0
Workpiece Fig. 1. Experimental setup for polishing tests.
Polishing pad
Rotational speed of tool Scanning speed of tool Polishing pressure Slurry Slurry supply rate
2.2. Mechanism of low removal rate at the beginning of polishing process
Removal rate mm3/h
The trend of low removal rate at the beginning of polishing process, as previously described, is commonly observed in fabrication processes. This part of the process is referred to as “dummy polishing”, and it considerably increases the production cost. The material removal rate at the beginning of polishing process is well known to be deteriorated
9 8
Variation rate 10%
7 Max. variation rate 28%
5
0
1
2
8
9
3 4 5 6 7 Polishing session
Pore
Plateau
4 3 0
3 4 5 6 7 Polishing session
by burrs on the surface of polishing pad generated during the surface conditioning process [19]. Fig. 3 shows the burr area on polishing pad surface before each polishing session, as determined via image analysis of laser scanning microscope (VK-X260; Keyence Corp.) images. We evaluated the area where the surface was higher than the most frequent height in surface height map of the entire polishing pad by more than 15 μm as the burr area. The large area on pad surface was covered with the burrs before first polishing session due to the surface conditioning with a diamond disk, and then almost all the burrs immediately disappeared during first session. This result suggests that burr generation was not responsible for the low removal rate during the first half of the polishing process (Fig. 2). We estimated the number of abrasive particles on the polishing pad surface to investigate the reason for low material removal rate at the beginning of the process. The surface of the polishing pad comprised pores ranging from 30 to 500 μm (average: 160 μm) in diameter and plateaus between each pore (Fig. 4). The abrasive particles held on the plateau area are able to remove material from the workpiece, that is, they can act as the active abrasive particles. We determined the plateau area with the abrasive particles using scanning electron microscopy (SEM; TM3000; Hitachi HighTechnologies Corp.) images of the polishing pad surface after each polishing session. Fig. 5(a) shows the SEM image of the pad surface after the final polishing session. White spots in the image represent remaining material, which was confirmed to comprise only abrasive particles by energy-dispersive X-ray spectrometry (EDX; S-3000; Hitachi High-Technologies Corp.) analysis. The area with higher brightness on the plateau in the images was evaluated as the plateau area with the abrasive particles. Fig. 5(b) shows the ratio of the plateau area with the abrasive particles to the pad surface area after each polishing session. The plateau area with the abrasive particles was small during the first half of the polishing process, particularly, during the first and second sessions (Fig. 5(b)), corresponding to the result of the material removal rate (Fig. 2). To investigate the small number of the active abrasive particles at the beginning of the polishing process, we estimated the number of abrasive particles on the polishing pad surface existing before each
Synthetic quartz glass Diameter: 100 mm Polyurethane foam-polishing pad with cerium dioxide particles Outer diameter: 6 mm; inner diameter: 2 mm 10,000 rpm 10 mm/s 195 kPa Ceria (mean diameter: 1 μm), 1 wt% 500 mL/min
which were measured using an optical profiler (NewView 200; Zygo Corp.). The 10-min polishing session was continuously repeated eight times in the series of polishing process. To remove glass chips generated during polishing session, the surface of polishing pad was cleaned by a nylon brush before the next session. Fig. 2 shows a plot of the changes in the material removal rate vs. polishing session. The material removal rate dramatically changed as polishing progressed. The maximum variation in the removal rate was 28%; however, manufacturers require < 10% variance (grey shaded area in Fig. 2). The low removal rate during the first half of the polishing process, particularly, during the first and second sessions and the sudden decrease in the removal rate during the final session significantly deteriorated stability of the material removal rate.
6
2
Fig. 3. Changes in the burr area on polishing pad surface before polishing session.
Table 1 Glass polishing conditions. Workpiece
1
8
9 200
Fig. 2. Changes in material removal rate with accumulated polishing sessions using a commercially available polishing pad.
Fig. 4. Optical microscope image of the surface of a commercial polishing pad. 2
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Abrasive particles Polishing pad
Abrasive particle
1 mm Polishing pad
(a) Scanning electron microscopy image of the surface of a commercial polishing pad after the final polishing session
Area ratio of abrasive %
Workpiece
14 12 10 8 6 4 2 0
Pore area
Plateau area
Active abrasive particle
(a) Model of behavior of an abrasive particle on polishing pad surface
At the beginning of polishing process
0
1
2
3 4 5 6 7 Polishing session
8
9 After polishing process progressed
(b) Changes in the ratio of the plateau area with the abrasive particles after polishing session using a commercial polishing pad
Area ratio of abrasive %
Fig. 5. Surface conditions of a commercial polishing pad after polishing session.
14 12 10 8 6 4 2 0
(b) Mechanism of the low material removal rate at the beginning of polishing process Fig. 7. Mechanism of material removal in polishing process.
Pore
mechanism of the low material removal rate at the beginning of polishing process. Slurry supplied on polishing pad surface has difficulties in penetrating into the contact region between the plateau and the workpiece surface because of the considerably small gap between the surfaces; hence, most of the abrasive particles cannot be held directly on the plateau area. As illustrated in Fig. 7(a), they are held on the pore areas first. Then, some of them are transported from the pore area to the contact region. Finally, they are held on the plateau area and act as the active abrasive particles. At the beginning of polishing process, as illustrated in Fig. 7(b), only a few abrasive particles exist on the pore area. A small number of the abrasive particles, thus, can be transported to the contact region between the plateau and the workpiece surface, resulting in low material removal rate. As the polishing process progresses, the pore area becomes filled with abrasive particles, and then a large number of the particles can be transported to the contact region, leading to high material removal rate.
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Fig. 6. Changes in the ratio of the plateau area and the pore area with the abrasive particles before polishing session using a commercial polishing pad.
polishing session. Fig. 6 plots the ratio of the plateau area (green dots) and the pore area (red dots) with the abrasive particles to the pad surface area before each polishing session. As revealed by a comparison of the results for the plateau area after session (Fig. 5(b)) and before session (green dots in Fig. 6), the abrasive particles held on the plateau area during polishing session were almost completely removed by brushing process before the next session. Thus, there is little difference in the initial number of the abrasive particles on the plateau area between any eight polishing sessions (green dots in Fig. 6). On the other hand, the abrasive particles were accumulated on the pore area as the polishing process progressed (red dots in Fig. 6). The number of abrasive particles on the pore area existing before session was small at the beginning of polishing process and increased gradually, as with the number of the active abrasive particles (Fig. 5(b)). Based on the aforementioned results, we propose the following
2.3. Mechanism of sudden decrease in removal rate during polishing process The material removal rate dropped by 18% during the final session of the polishing process (Fig. 2). A large number of studies have reported that surface of polishing pad become worn and glazed over time during polishing process, which leads to decrease in material removal rate [5,7]. Fig. 8 shows a plot of the glazed area on the polishing pad surface after each polishing session. The glazed area was measured in terms of the number of pixels that were glazed, that is, reflected larger quantity of laser beam in images of the polishing pad surface measured by a laser scanning microscope. The dramatic increase in the glazed 3
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Fig. 8. Changes in the glazed area on polishing pad surface after polishing session.
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Fig. 12. Changes in material removal rate with accumulated polishing sessions using a newly developed polishing pad.
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Fig. 13. Changes in the ratio of the plateau area with the abrasive particles after polishing session using a newly developed polishing pad.
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Fig. 14. Changes in the ratio of the plateau area and the pore area with the abrasive particles before polishing session using a newly developed polishing pad.
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The glazed pad surface was indicated to have negligible effect on removal rate also from the result during the first half of the polishing process, where both the removal rate (Fig. 2) and the glazed area on the pad surface (Fig. 8) gradually increased. We performed EDX analysis of the polishing pad surface after the final session to investigate the sudden decrease in removal rate. The result of the EDX analysis and laser scanning microscope observations of the pad surface revealed that huge workpiece debris (height of
Fig. 11. Wettability of the polishing pad surfaces.
area was not observed during the final polishing session, which indicates that the sudden decrease in the removal rate during the final session (Fig. 2) was not caused by the glazed surface of polishing pad. 4
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unlike in the case of the commercial pads (Fig. 5(b)). These results indicate that the abrasive particles supplied on the newly developed pad surface were held directly on the plateau area. The 13% variation in the material removal rate (Fig. 12) was, however, still higher than the industry demand of 10%. As shown in Fig. 12, the stability of the removal rate was deteriorated by the decrease in the removal rate during the third polishing session. The observations of the polishing pad surface revealed that it was attributed to the adhesion of tiny workpiece fragments to the pad surface. To enhance the self-cleaning effect of the developed polishing pad surface by promoting the penetration of slurry into the contact region between the plateau on pad surface and the workpiece surface, we applied vibration at a 0.2-Hz frequency and a 19.5-kPa amplitude to the polishing tool in the direction normal to the workpiece surface. The vibration was generated by varying the length of the spring that maintains the polishing pressure. To minimize the direct influence of the varying polishing pressure on the removal rate, the polishing tool was scanned considering the phase difference of the vibration. Fig. 15 shows that the maximum variation in the removal rate was reduced to 8% by a vibration-assisted polishing method with polishing pads containing TiO2 particles. The surface quality of the polished workpiece measured by an optical profiler was equivalent to that obtained by a conventional polishing method with commercial pads at 4nm P-V and 0.18-nm SRa in an area of 0.26 × 0.30 mm.
Max. variation rate 8%
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Fig. 15. Changes in material removal rate in vibration-assisted polishing using a newly developed polishing pad.
∼10 μm) were adhered to the pad surface. The polishing tool was confirmed to be pushed up by ∼10 μm during the final polishing session by the position data of the polishing tool along the axis normal to the workpiece surface measured using a laser displacement meter. This result indicates that the adhesion of huge workpiece debris to the pad surface can drastically decrease material removal rate by creating a considerably large gap between the pad surface and the workpiece surface.
4. Conclusions To suppress the variation in the material removal rate with the accumulated polishing time in polishing with small tools, a vibrationassisted polishing method using polishing pads with TiO2 particles was developed. Our experimental results showed that the proposed method significantly improved the stability of the material removal rate by promoting the penetration of slurry into the contact region between the polishing pad surface and the workpiece surface due to the high hydrophilicity of the TiO2 particles and the vibration of the polishing tool.
3. Newly developed polishing method To promote the penetration of slurry into the contact region between the plateau on polishing pad surface and the workpiece surface, we developed a new polishing pad with TiO2 particles as fillers. TiO2 particles are highly hydrophilic, which should enhance the wettability of the pad surface. The penetration of more slurry into the contact region enables the abrasive particles to be held directly on the plateau area and prevents strong adhesion of workpiece debris to the pad surface by self-cleaning effect [20]. Rutile TiO2 spherical particles (TITANIX JR-603; TAYCA Corp.; mean diameter = 0.3 μm, TiO2 > 90%) were contained at 10 wt% in a polyurethane-foam-type polishing pad. Fig. 9 shows the surface of the developed polishing pad with exposed pores with diameters ranging from 30 to 800 μm (average: 170 μm). Observations of the plateau on the pad surface indicate that the developed polishing pads have slightly different surface textures and approximately same surface roughness compared with those of the commercial polishing pads (Fig. 10). The type-C rubber hardness of the developed pad measured using a durometer (GD-703 N; TECLOCK Corp.) is identical to that of the commercial pad at 55. Water-contact-angle measurements performed using a contact angle meter (DM-300; Kyowa Interface Science Co., Ltd.) confirms that the developed pad surface is considerably more hydrophilic than the commercial pad surface (Fig. 11). As shown in Fig. 12, the developed polishing pads with TiO2 particles solved the problems associated with low removal rate at the beginning of polishing process and sudden decrease in removal rate induced by adhesion of huge workpiece debris. Consequently, the maximum variation in the removal rate was suppressed to 13%. Fig. 13 shows the plateau area with the abrasive particles after each polishing session and Fig. 14 shows the plateau area (green dots) and the pore area (red dots) with the abrasive particles before each session. The number of the abrasive particles on the pore area before session was small at the beginning of polishing process (red dots in Fig. 14) as in the case of using the commercial pads (red dots in Fig. 6). On the other hand, many abrasive particles were observed on the plateau area after session from the beginning of the polishing process (Fig. 13)
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