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Grinding characteristics of micro-abrasive pellet tools fabricated by a LIGA-like process
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 212–219
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Grinding characteristics of micro-abrasive pellet tools fabricated by a LIGA-like process S.Y. Luo a,b,, T.H. Yu a, C.Y. Liu c, M.H. Chen a,d a
Department of Mechatronic Engineering, Huafan University, Taiwan Advanced Manufacturing Research Center, Huafan University, Taiwan Department of Information Technology and Communication, Tungnan University, Taiwan d Department of Marine Engineering, Taipei College of Maritime Technology, Taiwan b c
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 August 2008 Received in revised form 17 November 2008 Accepted 19 November 2008 Available online 6 December 2008
The purpose of this paper was to investigate the wearing and grinding characteristics of the microabrasive pellet tools with 4–6 mm diamond particles fabricated by a LIGA-like process that has microlithography with photoresist mold and nickel/diamond composite electroforming. The results showed that when the micro-pellet tool containing partial resist joint with a root on the substrate was designed and fabricated, the tool against alumina sandpaper in wear test showed lesser amount of micro-pulledout pellets than the tools with flat joint type, which displayed the tool to have better adhesion strength. In addition, when the micro-diamond tools were used to grind silicon wafers, the surface appearance of wafers showed ductile behavior. The surface roughness of wafers ground with increased pellet tool rotation speed became better and Ra ¼ 0.05 mm was achieved. & 2009 Elsevier Ltd. All rights reserved.
Keywords: LIGA like Micro-diamond abrasive tool Wear test Grinding
1. Introduction LIGA-like technology has been widely used for development of innovative new products such as micro-molds [1–4], microactuators and sensors [5,6], micro-lens, micro-flow channels [7], etc. In this paper, a new micro-diamond abrasive pellet array tool developed by a LIGA-like process was used to grind silicon wafers and then study their grinding characteristics. Zhou et al. [8] studied an advanced manufacturing method for silicon wafers, using fixed abrasive instead of free slurry, to provide excellent surface roughness and global flatness. The denser cutting path contributes to surface roughness improvement, but an excessive cutting path density often leads to sever burn marks on the wafer surface. To¨nshoff et al. [9] stated that in the face tangential grinding of silicon wafer using a cup grinding wheel the diamond grit size has large influence on surface quality and depth of damage. The use of finer diamond grit of 15 mm leads to improvement in surface roughness and damage of approximately 60% compared to coarser diamond grit of 46 mm. Even though some studies related to the grinding performance of silicon wafers [10–12] have been carried out, in this work, using
Corresponding author at: Department of Mechatronic Engineering, Huafan University, Taiwan. E-mail address: [email protected] (S.Y. Luo).
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.11.007
a new developed micro-abrasive pellet tool, we tried to study the wearing behavior and grinding characteristics of silicon wafers.
2. Experiment process 2.1. Fabrication of micro-diamond abrasive pellet tool The micro-diamond abrasive pellet tool design is schematically shown in Fig. 1. This tool can provide better chip removability and better heat dissipation, which can improve loading and overheat during micro-grinding. Its manufacturing procedure using LIGAlike technology as shown in Fig. 2 mainly included three steps [13]: (1) micro-lithography that contains the design of the photo mask, resist coating, UV light exposure, and development, (2) electrolytic machining that make holes on substrates, and (3) micro-composite electroforming that deposits diamond particles (size of 4–6 mm) into the nickel matrix on substrates using the nickel sulfamate bath. Besides, the substrate surface needs a very good pretreatment and copper sputtering [13] or zincating [14] to enhance adhesion. Two different micro-diamond/nickel pellets containing a flat joint type without electrolytic machining and a partial resist joint type with a root of electrolytic machining on the T6061 Al-alloy substrate designed as shown in, respectively, Figs. 3(a) and (b) were investigated for their performances. Figs. 3(c) and (d) display the appearance of micro-pellets on the tool with a hexagonal
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shape and a round shape of diameter 200 mm and height 100 mm corresponding to Figs. 3(a) and (b), respectively. The micropellet’s pitch was 400 mm. This shows that LIGA process can easily produce a specific geometrical shape. 2.2. Wear test
213
were measured using a tool maker’s microscope. The five measured positions on the tool surface were located at the circumference 901 apart and the center, denoted by 1, 2, 4, 5, and 3, respectively.
2.3. Grinding test
Wear test for micro-diamond abrasive pellet tool was used to evaluate the adhesive strength between the pellets and the substrate and wear resistance. A small lapping machine in wear test was employed to study the micro-pellet tool against alumina sandpaper of 1000 mesh size. The applied pressure was 156–785 kPa and the rotation speed of table was 50 rpm. The wear time was set at 60 s and water was used as the cutting fluid. The pulled-out micro-pellets and the wear height of micro-pellet
Diamond /nickel abrasive pellet
Photoresist
Fig. 1. A schematic micro-nickel/diamond abrasive pellet tool.
Flat grinding test of silicon wafers was used to investigate the performance of micro-abrasive pellet tool. The vertical CNC machining center with a rotation table using a pre-compressed tool (Fig. 4) was operated and its engagement of kinematics in vertical flat grinding is shown in Fig. 5. The operating conditions in the tests are given in Table 1. A micro-abrasive pellet tool of nickel bond with diamond of size 4–6 mm was fabricated. The tool diameter was 30 mm and the diameter and height of diamond/ nickel pellets were 200 and 100 mm, respectively (Fig. 1). The feed rate of the tool in the flat grinding was 5 mm/min and the total depth of feed was 5–25 mm. The surface roughness of silicon wafer ground by micro-abrasive pellet tool under several spindle speeds of 50–1000 rpm was investigated. A stylus profilometer (Taylor Hobson) was used to investigate the surface roughness of the ground silicon wafer at an evaluation length of 8 mm and cut-off of 0.8 mm. Each measurement that took perpendicular to grinding traces at 901, 1801, 2701, and 3601 of the ground wafer was repeated three times and the mean values for Ra and Rmax were calculated. Using a SEM and a toolmaker’s microscope the diamond wear conditions and the trace characteristics of the silicon wafer, respectively, were studied.
Photoresist Electrolytic hole Substrate
UV Light Photo mask
Fig. 2. The manufacturing procedure of micro-abrasive pellet tool: (a) spin coating of JSR resist, (b) UV exposure, (c) development, (d) electrolytic machining, (e) composite electroforming, and (f) removal partial resist.
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Diamond/nickel pellets
Spindle Micro abrasive pellet tool
Silicon wafer
Al substrate
Fixture
Rotation table
Diamond/nickel pellets Resist
CNC X-Y table
Root
Al substrate
Al substrate
Wafer
Resist layer
Micro tool
Fig. 5. (a) A schematic vertical flat grinding and (b) its engagement of kinematics.
Table 1 Operating conditions in the grinding test.
Fig. 3. Joint type of micro-abrasive pellets on the substrate: (a) flat and (b) partial resist with root. The appearance of micro-abrasive pellets on the substrate for (c) flat joint and (d) partial resist.
Grinding machine Tool Micro-pellet Diamond size Bond type Spindle speed Vs Feed rate Vf Table speed Vt Total depth of grinding d Workpiece material Coolant
Machining center + 30 mm + 200 mm 100 mm 4–6 mm Nickel 50,100, 200, 300, 400, 500, 1000 rpm 5 mm/min 50 rpm 5, 10, 15, 20, 25 mm Si wafer (111)4’’ Water (3.5 l/min)
50 Alumina sandpaper 1000 mesh Screw Al substrate Micro abrasive pellet tool Fig. 4. A schematic fixture of micro-nickel/diamond abrasive pellet tool with a pre-compressed spring.
3. Results and discussion
Average percentage of pulled-out micro pellets (%)
Spring
Flat joint
40
Partial resist joint 30
20
10
0 156
3.1. Strength and wear of micro-abrasive pellet tools Strength of the joint between the micro-pellets and the substrate matrix for abrasive tools is a very important factor in
313
470
627
785
Pressure (kPa) Fig. 6. Variation of the average percentage of pulled-out micro-pellets and the applied pressure for two different micro-pellet tools with flat joint type and partial resist joint type against sandpaper.
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215
Resist layer Pull-out
Fig. 7. Appearance of some pulled-out micro-pellets of tool with (a) flat joint type and (b) partial resist joint type after wear.
Pressure (kPa) 785
25
0.30
Alumina sandpaper 1000 mesh
Surface roughness, Ra (um)
Average wear height of micro pellets (um)
30
627 470
20
313 156
15 10
As-received
0.25
Spindle speed (rpm) virgin 50 100 200 300 400 500 1000
0.20 0.15 0.10 0.05 0.00 90°
5
180°
270°
360°
Position of ground wafer
0
1
2
3 Position
4
5
Fig. 8. Average wear height of micro-abrasive diamond pellets on the different positions of tool with flat joint type.
the grinding or lapping application. Hence, the micro-abrasive pellet tools fabricated by a LIGA process would be worn against the alumina sandpaper to evaluate the conditions of pulled-out micro-pellets. The percentage of the pulled-out micro-pellets is defined as the number of pulled-out micro-pellets divided by the total number of micro-pellets observed using a toolmaker’s microscope. The average percentage of pulled-out micro-pellets for two different of micro-tools with flat joint type (Fig. 3(a)) and partial resist joint type (Fig. 3(b)) under several pressures against a sandpaper is shown in Fig. 6. It can be seen that the percentage of pulled-out micro-pellets for the tool with flat joint type increases with pressure. At the applied pressure of 156 kPa during the wear test the pulled-out micro-pellets on the tool surface reached up to 17%. For the micro-pellet tool with a partial resist and root joint type, at the pressure of 785 kPa the pulled-out micro-pellets were only about 2%. This implies that using a partial resist and root joint between the micro-pellet and the substrate is feasible under severe grinding or lapping. Fig. 7(a) shows the appearance of some pulled-out micropellets of tool with flat joint type after wear. However, when
Surface roughness, Rmax (um)
0 3.00 As-received 2.50
Spindle speed (rpm) virgin 50 100 200 300 400 500 1000
2.00 1.50 1.00 0.50 0.00 90°
180°
270°
360°
Position of ground wafer Fig. 9. Average surface roughness (a) Ra and (b) Rmax on the different positions of silicon wafer produced by the micro-diamond abrasive pellet at 50–1000 rpm and a feed rate of 5 mm/min.
using the micro-diamond abrasive pellet tool containing a partial resist joint type with a root being employed, the percentage of pulled-out micro-pellets displayed with increasing applied pressure was very low, only 1–2%. A typical worn surface of micro-tool with partial resist joint type is shown in Fig. 7(b). This shows that the micro-diamond abrasive pellet tool with a partial resist and root joint type can have better adhesive strength of micro-pellets on the substrate. Besides, joint
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strength for different shapes of pellets with an equivalent area would show a similar result. The average wear height of micro-abrasive diamond pellets on the different positions of tool with flat joint type is shown in Fig. 8. It can be seen that the average wear height of microabrasive pellets increased with applied pressure. At a lower applied pressure of 156 kPa the wear height of the micro-pellets was about only 1–3 mm. When the applied pressure reached 785 kPa, the wear height of micro-pellets increased to about 14 mm. In addition, the average wear height of micro-abrasive diamond pellets at different positions of tool showed a small variation. This may be due to the slight difference of pellet’s height electroformed in the bath.
3.2. Surface roughness of silicon wafer The micro-abrasive diamond pellet tool was used to grind silicon to evaluate its performance. Figs. 9(a) and (b), respectively, show the surface roughness Ra and Rmax produced at a spindle speed of 50–1000 rpm and feed rate of 5 mm/min before and after grinding of silicon wafer by the micro-diamond abrasive pellet tool. From the figure, the roughness Ra of as-received silicon wafer
surface was about 0.22 mm and Rmax was 2.0–2.23 mm. When a spindle speed of 50 rpm was used to grind silicon wafer, the roughness Ra of silicon wafer obtained reduced to about 0.16 mm and Rmax to 1.50–1.75 mm, and they reduced with spindle speed. However, when the micro-diamond abrasive tool was operated at 400, 500, or 1000 rpm to grind the silicon wafer, the roughness value Ra of silicon wafer obtained varied from about 0.09 mm down to 0.05 mm and Rmax from 0.8 mm down to 0.6 mm. This is because at higher spindle speeds the grinding trace produced is denser, which causes the silicon wafer to obtain better surface roughness. But this improvement lessens at too high grinding speeds. In addition, variation of the roughness values with measuring positions was small. This variation is attributed to the slight difference of pellet’s height on the tool and tool fixture. The grinding trace on the wafer surface at 300 and 1000 rpm is shown in Figs. 10(a) and (b), respectively. From Fig. 10(b), it can be seen that some burn marks have appeared on the wafer surface. This may be because the grinding cut path density is too dense and the diamond grains produces dull, which causes the wafer overheat. Hence, using a higher spindle speed for microdiamond abrasive pellet tool can improve the wafer roughness, but too high speeds will produce an excessive grinding trace
Fig. 10. Grinding trace on the wafer surface at (a) 300 and (b) 1000 rpm.
Fig. 11. Typical wear appearance of micro-pellet tool with (a) flat joint type and (b) partial resist joint type.
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respectively. It can be seen that few chips exist on the tool surface after grinding silicon. This micro-pellet array construction can have good chip removal ability during grinding. The typical diamond grits on the pellet tool surface before grinding are shown in Fig. 12(a). It can be seen that many diamond grits of size 4–6 mm are exposed on the nickel matrix. Figs. 12(b) and (c) show the appearance of the worn diamond grits on the micro-pellets after grinding silicon wafer under 300 and 1000 rpm, respectively. In the figures, some diamond grits display flat condition, which implies that the tool cannot cut effectively and, in extreme cases, causes the wafer produce to burn. This may be due to the excessive cut path density producing too much friction under a vertical grinding kinematics at too high speeds. In addition, the appearance of the ground silicon wafers generated at the spindle speed of 50, 300, 500, and 1000 rpm is presented in Figs. 13(a)–(d), respectively. From the figures, it can be seen that most of the silicon wafer surfaces display ductile grinding traces, but have a few pits on the surface (Figs. 13(e) and (f)). Besides, the effect of the spindle down feed on the removal depth of silicon wafer is shown in Fig. 14 at the spindle speed of 300 rpm for the micro-abrasive pellet tool. It can be seen that when the micro-tool moved downward 5 mm for 1 min, the removal depth of silicon wafer was about only 2.5 mm. In the meantime, the produced spring pressure during the grinding was about 22 kPa (Fig. 15). Its removal efficiency was about 50%. This may be because the micro-pellet tool used is pre-compressed, which causes the spindle movement to be absorbed by the spring inside tool, thereby reducing the removal efficiency of the silicon wafer. However, when a larger down feed of the micro-tool was employed, a higher removal depth of wafer was obtained. For the feed of 20 mm with a spring pressure of 156 kPa (Fig. 15) the removal depth of wafer produced was about 15 mm with 75% efficiency. This shows that the more the feed of the micro-tool, the more the pressure produced, which leads to a larger material removal rate, but too large feed of tool will cause the grinding efficiency to reduce. Furthermore, the diamond grits on the micro-pellet tool fabricated by a LIGA-like process need to maintain a free cutting behavior, which improves its grinding efficiency.
4. Conclusion
Fig. 12. Typical worn diamond grits on the tool surface (a) before grinding, and after grinding of wafer at (b) 300 and (c) 1000 rpm.
density, which occasionally causes some burn marks on the wafer surface. However, the nickel bond hardness and the diamond grain type for the micro-diamond abrasive pellet tool need to be further studied as well.
3.3. Tool wear and silicon removal condition Figs. 11(a) and (b) show a typical worn appearance of micro-pellet tool with flat joint type and partial resist joint type,
The size and shape of micro-diamond abrasive pellets on the tool surface can be easily fabricated by a LIGA-like technology. Base on experiment results, when the construction of micro-diamond abrasive pellet tool designed containing a partial resist joint type with a root was fabricated, better adhesive strength of micro-pellets on the substrate was obtained and the pulled-out micro-pellets on the tool at a high applied pressure up to 785 kPa were still small, thereby being available under severe grinding or lapping. The microabrasive pellet tool produced was used to grind silicon wafer; a higher spindle speed lead to a better surface roughness, Ra ¼ 0.05 mm on the wafer, but at too high cutting speed its improvement lessened. Most of the silicon wafer surfaces displayed ductile grinding behavior, but had a few pits on the surface. Besides, when a larger down feed of the micro-tool was employed, higher removal depth of wafer was obtained, showing higher grinding efficiency. Furthermore, this micro-pellet array construction designed can have good chip removal ability during micro-grinding to maintain a free cutting behavior, which improves its grinding performance.
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Fig. 13. Typical ground silicon wafer surface at (a) 50, (b) 300, (c) 500, and (d) 1000 rpm, (e) magnification of (b), and (f) magnification of (c).
200 Spindle speed : 300rpm Table speed : 50rpm
15
150
Coolant : water
Pressure (kPa)
Removal depth of silicon wafer (um)
20
10
100
50
5
0
0 0
5
10
15 Feed (um)
20
25
Fig. 14. Relationship of the removal depth rate of silicon wafer and the spindle feed.
0
5
10
15
20
25
Feed (um) Fig. 15. Relationship of the spring pressure and the spindle feed.
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Acknowledgments The authors are thankful to the National Science Council of Taiwan for supporting this study under Contracts NSC 95-2221-E211-016 and NSC 96-2628-E-211-018-MY2. References [1] H. Yang, C.T. Pan, M.C. Chou, Ultra-fine machining tool/molds by LIGA technology, J. Micromech. Microeng. 11 (2001) 94–99. [2] R.D. Chien, Micromolding of biochip devices designed with microchannels, Sensors Actuators A 128 (2006) 238–247. [3] W.B. Young, Analysis of filling distance in cylindrical microfeatures for microinjection molding, Appl. Math. Model. 31 (2007) 1798–1806. [4] C.H. Wu, C.H. Chen, K.W. Fan, W.S. Hsu, Y.C. Lin, Design and fabrication of polymer microfluidic substrates using the optical disc process, Sensors Actuators A 139 (2007) 310–317. [5] B.Y. Shew, C.H. Kuo, Y.C. Huang, Y.H. Tsai, UV-LIGA interferometer biosensor based on the SU-8 optical waveguide, Sensors Actuators A 120 (2005) 383–389.
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[6] M. Matteucci, F. Pe´renne`s, B. Marmiroil, P. Miotti, L. Vaccari, A. Gosparini, A. Turchet, E. Di Fabrizio, Compact micropumping system based on LIGA fabricated microparts, Microelectron. Eng. 83 (2006) 1288–1290. [7] S.C. Tzeng, W.P. Ma, Study of flow and heat transfer characteristics and LIGA fabrication of microspinnerets, J. Micromech. Microeng. 13 (2003) 670–679. [8] L.B. Zhou, H. Eda, J. Shimizu, State-of-the art technologies and kinematical analysis for one-stop finishing of + 300 mm Si wafer, J. Mater. Process. Technol. 129 (2002) 34–40. [9] H.K. To¨nshoff, W.v. Schmieden, I. Inasaki, W. Konig, G. Spur, Abrasive machining of silicon, Ann. CIRP 39/2 (1990) 463–471. [10] Z.J. Pei, A. Strasbaugh, Fine grinding of silicon wafers: designed experiments, Int. J. Mach. Tools Manufact. 42 (2002) 395–404. [11] I. Zarudi, L.C. Zhang, Effect of ultraprecision grinding on the microstructural change in silicon monocrystals, J. Mater. Process. Technol. 84 (1998) 149–158. [12] Z.J. Pei, S.R. Billingsley, S. Miura, Grinding induced subsurface cracks in silicon wafers, Int. J. Mach. Tools Manufact. 39 (1999) 1103–1116. [13] S.Y. Luo, T.H. Yu, Y.C. Hu, Fabrication of micro nickel/diamond abrasive pellet array lapping tools using a LIGA-like technology, J. Micromech. Microeng. 17 (2007) 1130–1138. [14] J.G. Jin, S.K. Lee, Y.H. Kim, Adhesion improvement of electroless plated Ni layer by ultrasonic agitation during zincating process, Thin Solid Films 466 (2004) 272–278.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Grinding characteristics of micro-abrasive pellet tools fabricated by a LIGA-like process S.Y. Luo a,b,, T.H. Yu a, C.Y. Liu c, M.H. Chen a,d a
Department of Mechatronic Engineering, Huafan University, Taiwan Advanced Manufacturing Research Center, Huafan University, Taiwan Department of Information Technology and Communication, Tungnan University, Taiwan d Department of Marine Engineering, Taipei College of Maritime Technology, Taiwan b c
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 August 2008 Received in revised form 17 November 2008 Accepted 19 November 2008 Available online 6 December 2008
The purpose of this paper was to investigate the wearing and grinding characteristics of the microabrasive pellet tools with 4–6 mm diamond particles fabricated by a LIGA-like process that has microlithography with photoresist mold and nickel/diamond composite electroforming. The results showed that when the micro-pellet tool containing partial resist joint with a root on the substrate was designed and fabricated, the tool against alumina sandpaper in wear test showed lesser amount of micro-pulledout pellets than the tools with flat joint type, which displayed the tool to have better adhesion strength. In addition, when the micro-diamond tools were used to grind silicon wafers, the surface appearance of wafers showed ductile behavior. The surface roughness of wafers ground with increased pellet tool rotation speed became better and Ra ¼ 0.05 mm was achieved. & 2009 Elsevier Ltd. All rights reserved.
Keywords: LIGA like Micro-diamond abrasive tool Wear test Grinding
1. Introduction LIGA-like technology has been widely used for development of innovative new products such as micro-molds [1–4], microactuators and sensors [5,6], micro-lens, micro-flow channels [7], etc. In this paper, a new micro-diamond abrasive pellet array tool developed by a LIGA-like process was used to grind silicon wafers and then study their grinding characteristics. Zhou et al. [8] studied an advanced manufacturing method for silicon wafers, using fixed abrasive instead of free slurry, to provide excellent surface roughness and global flatness. The denser cutting path contributes to surface roughness improvement, but an excessive cutting path density often leads to sever burn marks on the wafer surface. To¨nshoff et al. [9] stated that in the face tangential grinding of silicon wafer using a cup grinding wheel the diamond grit size has large influence on surface quality and depth of damage. The use of finer diamond grit of 15 mm leads to improvement in surface roughness and damage of approximately 60% compared to coarser diamond grit of 46 mm. Even though some studies related to the grinding performance of silicon wafers [10–12] have been carried out, in this work, using
Corresponding author at: Department of Mechatronic Engineering, Huafan University, Taiwan. E-mail address: [email protected] (S.Y. Luo).
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.11.007
a new developed micro-abrasive pellet tool, we tried to study the wearing behavior and grinding characteristics of silicon wafers.
2. Experiment process 2.1. Fabrication of micro-diamond abrasive pellet tool The micro-diamond abrasive pellet tool design is schematically shown in Fig. 1. This tool can provide better chip removability and better heat dissipation, which can improve loading and overheat during micro-grinding. Its manufacturing procedure using LIGAlike technology as shown in Fig. 2 mainly included three steps [13]: (1) micro-lithography that contains the design of the photo mask, resist coating, UV light exposure, and development, (2) electrolytic machining that make holes on substrates, and (3) micro-composite electroforming that deposits diamond particles (size of 4–6 mm) into the nickel matrix on substrates using the nickel sulfamate bath. Besides, the substrate surface needs a very good pretreatment and copper sputtering [13] or zincating [14] to enhance adhesion. Two different micro-diamond/nickel pellets containing a flat joint type without electrolytic machining and a partial resist joint type with a root of electrolytic machining on the T6061 Al-alloy substrate designed as shown in, respectively, Figs. 3(a) and (b) were investigated for their performances. Figs. 3(c) and (d) display the appearance of micro-pellets on the tool with a hexagonal
ARTICLE IN PRESS S.Y. Luo et al. / International Journal of Machine Tools & Manufacture 49 (2009) 212–219
shape and a round shape of diameter 200 mm and height 100 mm corresponding to Figs. 3(a) and (b), respectively. The micropellet’s pitch was 400 mm. This shows that LIGA process can easily produce a specific geometrical shape. 2.2. Wear test
213
were measured using a tool maker’s microscope. The five measured positions on the tool surface were located at the circumference 901 apart and the center, denoted by 1, 2, 4, 5, and 3, respectively.
2.3. Grinding test
Wear test for micro-diamond abrasive pellet tool was used to evaluate the adhesive strength between the pellets and the substrate and wear resistance. A small lapping machine in wear test was employed to study the micro-pellet tool against alumina sandpaper of 1000 mesh size. The applied pressure was 156–785 kPa and the rotation speed of table was 50 rpm. The wear time was set at 60 s and water was used as the cutting fluid. The pulled-out micro-pellets and the wear height of micro-pellet
Diamond /nickel abrasive pellet
Photoresist
Fig. 1. A schematic micro-nickel/diamond abrasive pellet tool.
Flat grinding test of silicon wafers was used to investigate the performance of micro-abrasive pellet tool. The vertical CNC machining center with a rotation table using a pre-compressed tool (Fig. 4) was operated and its engagement of kinematics in vertical flat grinding is shown in Fig. 5. The operating conditions in the tests are given in Table 1. A micro-abrasive pellet tool of nickel bond with diamond of size 4–6 mm was fabricated. The tool diameter was 30 mm and the diameter and height of diamond/ nickel pellets were 200 and 100 mm, respectively (Fig. 1). The feed rate of the tool in the flat grinding was 5 mm/min and the total depth of feed was 5–25 mm. The surface roughness of silicon wafer ground by micro-abrasive pellet tool under several spindle speeds of 50–1000 rpm was investigated. A stylus profilometer (Taylor Hobson) was used to investigate the surface roughness of the ground silicon wafer at an evaluation length of 8 mm and cut-off of 0.8 mm. Each measurement that took perpendicular to grinding traces at 901, 1801, 2701, and 3601 of the ground wafer was repeated three times and the mean values for Ra and Rmax were calculated. Using a SEM and a toolmaker’s microscope the diamond wear conditions and the trace characteristics of the silicon wafer, respectively, were studied.
Photoresist Electrolytic hole Substrate
UV Light Photo mask
Fig. 2. The manufacturing procedure of micro-abrasive pellet tool: (a) spin coating of JSR resist, (b) UV exposure, (c) development, (d) electrolytic machining, (e) composite electroforming, and (f) removal partial resist.
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Diamond/nickel pellets
Spindle Micro abrasive pellet tool
Silicon wafer
Al substrate
Fixture
Rotation table
Diamond/nickel pellets Resist
CNC X-Y table
Root
Al substrate
Al substrate
Wafer
Resist layer
Micro tool
Fig. 5. (a) A schematic vertical flat grinding and (b) its engagement of kinematics.
Table 1 Operating conditions in the grinding test.
Fig. 3. Joint type of micro-abrasive pellets on the substrate: (a) flat and (b) partial resist with root. The appearance of micro-abrasive pellets on the substrate for (c) flat joint and (d) partial resist.
Grinding machine Tool Micro-pellet Diamond size Bond type Spindle speed Vs Feed rate Vf Table speed Vt Total depth of grinding d Workpiece material Coolant
Machining center + 30 mm + 200 mm 100 mm 4–6 mm Nickel 50,100, 200, 300, 400, 500, 1000 rpm 5 mm/min 50 rpm 5, 10, 15, 20, 25 mm Si wafer (111)4’’ Water (3.5 l/min)
50 Alumina sandpaper 1000 mesh Screw Al substrate Micro abrasive pellet tool Fig. 4. A schematic fixture of micro-nickel/diamond abrasive pellet tool with a pre-compressed spring.
3. Results and discussion
Average percentage of pulled-out micro pellets (%)
Spring
Flat joint
40
Partial resist joint 30
20
10
0 156
3.1. Strength and wear of micro-abrasive pellet tools Strength of the joint between the micro-pellets and the substrate matrix for abrasive tools is a very important factor in
313
470
627
785
Pressure (kPa) Fig. 6. Variation of the average percentage of pulled-out micro-pellets and the applied pressure for two different micro-pellet tools with flat joint type and partial resist joint type against sandpaper.
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215
Resist layer Pull-out
Fig. 7. Appearance of some pulled-out micro-pellets of tool with (a) flat joint type and (b) partial resist joint type after wear.
Pressure (kPa) 785
25
0.30
Alumina sandpaper 1000 mesh
Surface roughness, Ra (um)
Average wear height of micro pellets (um)
30
627 470
20
313 156
15 10
As-received
0.25
Spindle speed (rpm) virgin 50 100 200 300 400 500 1000
0.20 0.15 0.10 0.05 0.00 90°
5
180°
270°
360°
Position of ground wafer
0
1
2
3 Position
4
5
Fig. 8. Average wear height of micro-abrasive diamond pellets on the different positions of tool with flat joint type.
the grinding or lapping application. Hence, the micro-abrasive pellet tools fabricated by a LIGA process would be worn against the alumina sandpaper to evaluate the conditions of pulled-out micro-pellets. The percentage of the pulled-out micro-pellets is defined as the number of pulled-out micro-pellets divided by the total number of micro-pellets observed using a toolmaker’s microscope. The average percentage of pulled-out micro-pellets for two different of micro-tools with flat joint type (Fig. 3(a)) and partial resist joint type (Fig. 3(b)) under several pressures against a sandpaper is shown in Fig. 6. It can be seen that the percentage of pulled-out micro-pellets for the tool with flat joint type increases with pressure. At the applied pressure of 156 kPa during the wear test the pulled-out micro-pellets on the tool surface reached up to 17%. For the micro-pellet tool with a partial resist and root joint type, at the pressure of 785 kPa the pulled-out micro-pellets were only about 2%. This implies that using a partial resist and root joint between the micro-pellet and the substrate is feasible under severe grinding or lapping. Fig. 7(a) shows the appearance of some pulled-out micropellets of tool with flat joint type after wear. However, when
Surface roughness, Rmax (um)
0 3.00 As-received 2.50
Spindle speed (rpm) virgin 50 100 200 300 400 500 1000
2.00 1.50 1.00 0.50 0.00 90°
180°
270°
360°
Position of ground wafer Fig. 9. Average surface roughness (a) Ra and (b) Rmax on the different positions of silicon wafer produced by the micro-diamond abrasive pellet at 50–1000 rpm and a feed rate of 5 mm/min.
using the micro-diamond abrasive pellet tool containing a partial resist joint type with a root being employed, the percentage of pulled-out micro-pellets displayed with increasing applied pressure was very low, only 1–2%. A typical worn surface of micro-tool with partial resist joint type is shown in Fig. 7(b). This shows that the micro-diamond abrasive pellet tool with a partial resist and root joint type can have better adhesive strength of micro-pellets on the substrate. Besides, joint
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strength for different shapes of pellets with an equivalent area would show a similar result. The average wear height of micro-abrasive diamond pellets on the different positions of tool with flat joint type is shown in Fig. 8. It can be seen that the average wear height of microabrasive pellets increased with applied pressure. At a lower applied pressure of 156 kPa the wear height of the micro-pellets was about only 1–3 mm. When the applied pressure reached 785 kPa, the wear height of micro-pellets increased to about 14 mm. In addition, the average wear height of micro-abrasive diamond pellets at different positions of tool showed a small variation. This may be due to the slight difference of pellet’s height electroformed in the bath.
3.2. Surface roughness of silicon wafer The micro-abrasive diamond pellet tool was used to grind silicon to evaluate its performance. Figs. 9(a) and (b), respectively, show the surface roughness Ra and Rmax produced at a spindle speed of 50–1000 rpm and feed rate of 5 mm/min before and after grinding of silicon wafer by the micro-diamond abrasive pellet tool. From the figure, the roughness Ra of as-received silicon wafer
surface was about 0.22 mm and Rmax was 2.0–2.23 mm. When a spindle speed of 50 rpm was used to grind silicon wafer, the roughness Ra of silicon wafer obtained reduced to about 0.16 mm and Rmax to 1.50–1.75 mm, and they reduced with spindle speed. However, when the micro-diamond abrasive tool was operated at 400, 500, or 1000 rpm to grind the silicon wafer, the roughness value Ra of silicon wafer obtained varied from about 0.09 mm down to 0.05 mm and Rmax from 0.8 mm down to 0.6 mm. This is because at higher spindle speeds the grinding trace produced is denser, which causes the silicon wafer to obtain better surface roughness. But this improvement lessens at too high grinding speeds. In addition, variation of the roughness values with measuring positions was small. This variation is attributed to the slight difference of pellet’s height on the tool and tool fixture. The grinding trace on the wafer surface at 300 and 1000 rpm is shown in Figs. 10(a) and (b), respectively. From Fig. 10(b), it can be seen that some burn marks have appeared on the wafer surface. This may be because the grinding cut path density is too dense and the diamond grains produces dull, which causes the wafer overheat. Hence, using a higher spindle speed for microdiamond abrasive pellet tool can improve the wafer roughness, but too high speeds will produce an excessive grinding trace
Fig. 10. Grinding trace on the wafer surface at (a) 300 and (b) 1000 rpm.
Fig. 11. Typical wear appearance of micro-pellet tool with (a) flat joint type and (b) partial resist joint type.
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respectively. It can be seen that few chips exist on the tool surface after grinding silicon. This micro-pellet array construction can have good chip removal ability during grinding. The typical diamond grits on the pellet tool surface before grinding are shown in Fig. 12(a). It can be seen that many diamond grits of size 4–6 mm are exposed on the nickel matrix. Figs. 12(b) and (c) show the appearance of the worn diamond grits on the micro-pellets after grinding silicon wafer under 300 and 1000 rpm, respectively. In the figures, some diamond grits display flat condition, which implies that the tool cannot cut effectively and, in extreme cases, causes the wafer produce to burn. This may be due to the excessive cut path density producing too much friction under a vertical grinding kinematics at too high speeds. In addition, the appearance of the ground silicon wafers generated at the spindle speed of 50, 300, 500, and 1000 rpm is presented in Figs. 13(a)–(d), respectively. From the figures, it can be seen that most of the silicon wafer surfaces display ductile grinding traces, but have a few pits on the surface (Figs. 13(e) and (f)). Besides, the effect of the spindle down feed on the removal depth of silicon wafer is shown in Fig. 14 at the spindle speed of 300 rpm for the micro-abrasive pellet tool. It can be seen that when the micro-tool moved downward 5 mm for 1 min, the removal depth of silicon wafer was about only 2.5 mm. In the meantime, the produced spring pressure during the grinding was about 22 kPa (Fig. 15). Its removal efficiency was about 50%. This may be because the micro-pellet tool used is pre-compressed, which causes the spindle movement to be absorbed by the spring inside tool, thereby reducing the removal efficiency of the silicon wafer. However, when a larger down feed of the micro-tool was employed, a higher removal depth of wafer was obtained. For the feed of 20 mm with a spring pressure of 156 kPa (Fig. 15) the removal depth of wafer produced was about 15 mm with 75% efficiency. This shows that the more the feed of the micro-tool, the more the pressure produced, which leads to a larger material removal rate, but too large feed of tool will cause the grinding efficiency to reduce. Furthermore, the diamond grits on the micro-pellet tool fabricated by a LIGA-like process need to maintain a free cutting behavior, which improves its grinding efficiency.
4. Conclusion
Fig. 12. Typical worn diamond grits on the tool surface (a) before grinding, and after grinding of wafer at (b) 300 and (c) 1000 rpm.
density, which occasionally causes some burn marks on the wafer surface. However, the nickel bond hardness and the diamond grain type for the micro-diamond abrasive pellet tool need to be further studied as well.
3.3. Tool wear and silicon removal condition Figs. 11(a) and (b) show a typical worn appearance of micro-pellet tool with flat joint type and partial resist joint type,
The size and shape of micro-diamond abrasive pellets on the tool surface can be easily fabricated by a LIGA-like technology. Base on experiment results, when the construction of micro-diamond abrasive pellet tool designed containing a partial resist joint type with a root was fabricated, better adhesive strength of micro-pellets on the substrate was obtained and the pulled-out micro-pellets on the tool at a high applied pressure up to 785 kPa were still small, thereby being available under severe grinding or lapping. The microabrasive pellet tool produced was used to grind silicon wafer; a higher spindle speed lead to a better surface roughness, Ra ¼ 0.05 mm on the wafer, but at too high cutting speed its improvement lessened. Most of the silicon wafer surfaces displayed ductile grinding behavior, but had a few pits on the surface. Besides, when a larger down feed of the micro-tool was employed, higher removal depth of wafer was obtained, showing higher grinding efficiency. Furthermore, this micro-pellet array construction designed can have good chip removal ability during micro-grinding to maintain a free cutting behavior, which improves its grinding performance.
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Fig. 13. Typical ground silicon wafer surface at (a) 50, (b) 300, (c) 500, and (d) 1000 rpm, (e) magnification of (b), and (f) magnification of (c).
200 Spindle speed : 300rpm Table speed : 50rpm
15
150
Coolant : water
Pressure (kPa)
Removal depth of silicon wafer (um)
20
10
100
50
5
0
0 0
5
10
15 Feed (um)
20
25
Fig. 14. Relationship of the removal depth rate of silicon wafer and the spindle feed.
0
5
10
15
20
25
Feed (um) Fig. 15. Relationship of the spring pressure and the spindle feed.
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