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Multi-wire sawing of sapphire crystals with reciprocating motion of electroplated diamond wires
CIRP Annals - Manufacturing Technology 62 (2013) 335–338
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Multi-wire sawing of sapphire crystals with reciprocating motion of electroplated diamond wires Hyoungjae Kim b, Doyeon Kim b, Chulmin Kim b, Haedo Jeong (2)a,* a b
Graduate School of Mechanical Engineering, Pusan National University, Republic of Korea Net Shape Research Group, Korea Institute of Industrial Technology, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting tool Wafer Multi-wire sawing
This study examined the multi-wire sawing of C-plane sapphire ingots using diamond wires. Feeding new wire during the reciprocating motion of the wire was found to vary the cutting force, wafer shape, and roughness as a result of the break-in effect. The break-in and wire wear seemed to cause a gradual change in the cutting performance along the ingot position. The cutting force results indicated that an inappropriate supply of wire yielded an unbalanced force between the front and back sides of the ingot, which was caused by a difference in the cutting depth along the ingot. The results showed that controlling the wire consumption resulted in an average flatness of 16 mm, with a maximum value of 26 mm. ß 2013 CIRP.
1. Introduction Sapphire, GaN, and SiC are representative single crystals that have been increasingly used for LED and power device applications [1–3]. It is well known that these crystals are difficult-to-machine materials because of their extreme hardness and the limited selection of tool materials [4]. Because of the increasing use of these materials, a multi-wire sawing (MWS) process is essential for the mass production of wafers with precise shapes by cutting the grown ingots into multiple sliced wafers. The MWS process is the most critical process to determine the final shape accuracy of a wafer. The conventional MWS process uses SiC abrasives suspended in a glycol-based lubricant [5]. However, sawing with a SiC slurry generates a large amount of waste and is ineffective for cutting hard crystals. For higher material removal efficiency and the reduction of waste, diamond wire has been adopted for cutting sapphire, SiC, and GaN materials [6,7]. However, precision shape control is still a problem because of the hardness, tool wear, and excessive wire deflection of diamond wire caused by inappropriate cutting conditions. Although diamond wire has been around for a long time, its application to the wafering process is relatively new, and its machinability has never required such a high level of shape accuracy for high-quality wafer production. During the wire sawing of sapphire ingots, wires move back and forth to provide better cutting performance through a higher material removal rate and more effective use of the diamond wires. In addition, as shown in Fig. 1, the wire or ingot is rocked during the cutting process to improve the removal rate by reducing the contact area between the ingot and the wires. The reciprocating motion of the wire has to be optimized by compensating for the wear of the diamond abrasives. Moreover, the feeding speed profile
* Corresponding author. 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.122
Fig. 1. Schematic view of wire sawing of sapphire wafer using diamond wire (left) and optical microscopic image of diamond wires with diameter of 180 mm and diamond size of 30–40 mm (right).
of an ingot into a diamond-coated wire must be considered to continuously compensate for the changing intersection areas of the sapphire ingot. In this study, the authors examined the effect of diamond-wire wear on various cutting performance variables of a C-plane sapphire, such as the roughness, thickness variation, and flatness. A tool dynamometer was used to measure the variation in the vertical feeding force, as well as the lateral cutting force, to better understand the material removal phenomenon in sapphire. 2. Wire motion and cutting profile 2.1. Reciprocation motion of wire In a conventional wire cutting process, uncoated piano wire runs one way at high speed in the presence of a uniformly distributed SiC slurry. Therefore, in the case of bare-wire MWS, the cutting abilities of the multiple wires are approximately the same over the multiple contact positions because of the evenly distributed cutting abrasives. However, when using diamondcoated wires in the MWS process, the wire repeatedly moves back and forth at high speed while progressively advancing by a given amount, defined as v0(tcf tcb) (Fig. 2). Therefore, the cutting
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
336
Fig. 2. Reciprocating motion of diamond wire during MWS process. tcy is the cycle time, and tcf and tcb are the times for the constant feed forward and backward, respectively. Fig. 3. Experimental setup for cutting force measurement.
ability of the diamond-coated wire degrades along the wire position because the sharp edges of the diamond abrasives are gradually worn down by repeated contacts. Thus, it is important to predict the total number of contacts at a fixed location for the wire. Therefore, the cutting ability of diamond coated wire degrades along the wire position since the sharp edges of diamond abrasives gradually worn by repeated contacts. So, it is important to predict the total number of contact at fixed location of a wire.
dynamometer had sensors embedded in four different locations, which could be used to obtain spatial information about the cutting force. Through the dynamometer measurements, the forces at the front and backside positions of the ingot could be analyzed to compare the cutting performances of new and used wires.
4. Results and discussion
2.2. Feed speed of ingot
4.1. Effect of wire break-in and wear on cutting performance
The ingot feed speed is a critical factor to achieve accurate wafer shape characteristics such as the flatness and total thickness variation (TTV). The material removal rate is inversely proportional to the cross-sectional area. Thus, the feed speed profile for a circular ingot can be simply calculated based on its diameter change according to the feeding direction. Therefore, the feed speed used in this experiment can be expressed as a function of wafer radius R and proportional constant k, as written in Eq. (1).
Diamond abrasives are electroplated on a wire using a nickel layer. Although this nickel layer ensures that the diamond abrasives are tightly secured to the wire surface, the layer on top of the abrasives suppresses the cutting performance because it prevents the diamonds from contacting the workpiece. Therefore, this layer should be properly dressed or broken-in to ensure higher cutting performance. An experiment that involved cutting a 4-in sapphire was performed to determine the effect of the break-in on the cutting performance, using a wire consumption of 106 m/wafer and a wire speed and tension of 400 m/min and 35 N, respectively. Fig. 4 shows the thickness variation at different wafer locations. The results clearly show that the thickness of the wafer increased as the wafer number increased. This increase in wafer thickness implies that the kerf loss decreased as a result of a decrease in the equivalent diameter of the cutting edges in the wire.
k VðzÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 R2 ðR zÞ2
(1)
3. Experimental setup 3.1. Multi-wire saw setup
855
average thickness ( m)
Experiments were performed using a production-scale multiwire saw machine, which had a maximum wire speed and tension of 700 m/min and 40 N, respectively. The diamond wire used in the experiment had a core diameter of 0.18 mm, and the attached diamond size was 30–40 mm. An experiment to investigate the break-in effect was performed by supplying a large amount of new wire to clearly see the tool wear transition. In addition, the wire consumption was varied from 24 to 12.3 m/wafer to determine the effect of the wire consumption on the cutting force deviation, as shown in Table 1.
850 845 840 835 830
Table 1 Experimental conditions.
0
Parameters
Conditions
Ingot Wire saw Main roller distance Wire speed Wire tension Wire consumption Cutting fluid Wire
4 in. single crystal sapphire, C-plane 470 mm 400, 500, 700 m/min 35, 40 N 106, 24, 18, 12.3 m/wafer 5 wt% concentration diluted with tap water D0.18 mm piano steel, 30–40 mm diamond
3.2. Tool dynamometer setup During the experiment, an ingot was attached to a dynamometer using an epoxy-based wax. A piezoelectric sensor was used to measure the dynamic cutting force. As shown in Fig. 3, the
10
20
30
40
50
60
wafer No. Fig. 4. Wafer thickness variation across ingot position.
It should be noted that in the initial 5–6 wafers, the thickness rapidly decreased with increasing wafer number, after which the thickness increased with the wafer number. This phenomenon is frequently observed in the diamond wire sawing process, and those initial wafers are usually related to the yield loss. It seems that this phenomenon involves two causes. The first is the break-in effect for large abrasives composed of large diamonds or agglomerated abrasives on a wire. In this stage, the highest spots of the covered nickel layer are rapidly removed and the sharp edges of the diamonds are exposed to increase the cutting performance by repeated contact with the sapphire.
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
Therefore, the high spots on a wire will remove a larger quantity of material around the front side of the ingot, which produces thin wafers near the feeding area. Spots that are higher than the average height are then rapidly worn down, and the diametric deviation is rapidly stabilized by the concentrated wear of those abrasives. The other phenomenon is the gradual wear of the diamond abrasives, which reduces the diameter of the wire and causes the wafer thickness to increase, with less kerf loss. Fig. 5 shows the roughness variation trend across the wafer locations. The roughness is generally determined by the stochastic nature of wire sawing and is a function of the indentation depth and feed speed [8]. As shown in Fig. 5, the roughness gradually decreases with an increase in the wafer number. Considering the results from Fig. 4, the enhanced roughness near the backside of the ingot is clearly the result of the wear of the diamond wire. 900
Roughness, Ra (nm)
750
600
450
337
4.2. Effect of wire consumption on cutting force Fig. 7 shows the measurement results for the ingot feed force against a wire as a function of the wire deflection and given wire tension. The feed forces were measured using a dynamometer by feeding an ingot into stationary diamond wires. Considering the wire deflection and ingot position, the theoretical feed force f can be calculated as follows in Eq. (2).
d
f ¼ 2ðT 0 þ DTÞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L2 =4 þ d
(2)
where T0 is the given wire tension and DT is the additional tension from the elastic elongation of the wire caused by wire deflection. d and L are the wire deflection and wire length between the rollers, respectively. As shown in Fig. 7, the measured feed forces are slightly higher than the theoretically predicted one, which means that the tension of the wire wound inside the main rollers is higher than the controlled value. This increased tension is caused by the rotational friction and inertia of the guide rollers located between the tension control dancer and the main roller. The results showed that the feed force is a linear function of the tension and deflection. Therefore, if the feed force can be measured during cutting, the inprocess deflection of the wire can be evaluated.
300
2.0
25N_Expe 30N_Expe 35N_Expe 40N_Expe 25N_Pred 30N_Pred 35N_Pred 40N_Pred
1.8 150
1.6 1.4
5 10 15 20 25 30 35 40 45 50 55 60 65 Wafer No.
Fig. 5. Roughness variation across ingot positions caused by degradation of diamond abrasives, along with average values at five measurement points.
Feed force (N)
-5 0
1.2 1.0 0.8 0.6 0.4 0.2
The graph in Fig. 6 shows the sori value, and the picture above the graph depicts the actual wafer shape after the MWS process. The ‘‘sori’’ of the wafer is defined as the difference between the maximum and minimum distances to the front surface of a free, unclamped wafer from the least-squares reference plane of the front surface [9]. The wafers near the wire supply position show high sori values, and the value decreases as the wafer number increases. In addition, several wafers near the feed-in position show irregular shapes, which seem to be caused by the break-in effect. The high sori values near the inlet position seem to be caused by an unstable wire condition, where the surface condition of the wire rapidly changes as a result of the break-in of the nickel layer and transient wear of the high cutting edges. Therefore, the wire consumption and tool life should be properly selected for precise cutting results by minimizing the transition wear of the diamond wire.
Fig. 6. Wafer shapes after cutting process and sori values.
0.0 -0.2
0
1
2
3
4
5
Wire deflection (mm) Fig. 7. Measurement and prediction results for feeding force on a single wire as a function of wire deflection and wire tension.
Based on these results, the feed force was measured during the cutting of 4-in. sapphire ingots under various wire consumption conditions. Fig. 8(a)–(c) show the effect of the wire consumption on the feed force trend. As the ingot feeds into the wires, the feed force is gradually increased with an increase in the cross section of the ingot. Then, the force is reduced with a reduction in the diameter. However, the maximum force occurs after 50–70% of the feed position, and the maximum point moves back as the wire consumption is reduced. This result indicates that the wire deflection increases as the wire consumption decreases. The wire deflection is accumulated as a form of elastic energy in the wires until the ingot passes the maximum diameter. This accumulated elastic energy in the wires exerts a high contact force on the ingot. Therefore, faster material removal occurs in the last half of the ingot from the combination of the accumulated elastic energy in wire and the rapid reduction in the cross-sectional area of the ingot. Thus, the force curve rapidly drops after its maximum. In addition, Fig. 8(d) shows the ratio of the feed forces measured by the sensors located at the front and back sides of the ingot. These results indicate that the penetration depth of the wire into the ingot is different between the front and the back sides and increases when the wire consumption is smaller than a critical value. In this case, a nonuniform cutting depth seems to affect the cutting accuracy because in the case of a 12.3-m/wafer condition, the sori value was measured at over 100 mm. Fig. 9 shows the friction force trend during wire sawing under different wire consumption conditions. As shown in the graph, the
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
338 0.5
Z1(Back_R) Z2(Back_L) Z3(Front_L) Z4(Front_R)
0.3 0.2 0.1 0.0
20 40 60 80 Cutting Depth(%)
(a)wire supply: 24.0m/wafer 0.5
60 50
0
20
40 60 80 Cutting Depth(%)
100 120
10
130
0
Force (N)
Force Ratio (%)
140
0
5
10 15 20 25 Wafer No.
0
5
10
15
Wafer No.
20
25 0
5
10 15 20 25 Wafer No.
110
0.2
Fig. 10. Wafer shape trend according to wire consumption.
100
0.1
24.0m/wafer 18.0m/wafer 12.3m/wafer
90
0.0
0
20 40 60 80 Cutting Depth(%)
100
0
(c)wire supply: 12.3m/wafer
20 40 60 80 Cutting Depth (%)
100
(d)force ratio: front/back
Fig. 8. Results for feed force under various wire consumption conditions based on wire positions vs. cutting depth (a–c) and their force unbalance ratios between front and backside positions (d). (a) Wire supply: 24.0 m/wafer, (b) wire supply: 18.0 m/ wafer, (c) wire supply: 12.3 m/wafer and (d) force ratio: front/back. 0.24
0.24
12.3m/wafer
0.16 0.12 0.08 0.04 0.00 0.0 0.24
24.0m/wafer
0.20
Friction force (N)
0.20 Friction force (N)
30 20
120
0.3
0.2
0.4
0.6
0.8
1.0
1.2
0.16 0.12 0.08 0.04 0.00 0.0
1.4
Feed force (N) 18.0m/wafer
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Feed force (N) 0.15
0.20 0.16
COF (n/a)
Friction force (N)
40
(b)wire supply: 18.0m/wafer
Z1(Back_R) Z2(Back_L) Z3(Front_L) Z4(Front_R)
0.4
18m/wafer
0.2
0.0
100
24m/wafer
0.3
0.1
0
106m/wafer
70
Z1(Back_R) Z2(Back_L) Z3(Front_L) Z4(Front_R)
0.4 Force (N)
Force (N)
0.4
SORI (μm)
0.5
0.12 0.08
0.2
0.4
0.6
0.8
1.0
Feed force (N)
1.2
1.4
5. Conclusion This study examined the multi-wire sawing of single-crystal sapphire ingots to determine the effects of the wire conditions on the cutting results. The results showed that the break-in of a wire significantly affects the shape quality of the cut wafers. In particular, the initial wear of an electroplated diamond wire has a strong relationship with the quality of the wafers located near the wire inlet position. In addition, the flatness of the wafer can be controlled by controlling the wire consumption per wafer through minimizing the break-in zone and excessive wear. The results of this study are not only applicable to the precision sawing of sapphire ingots but can also be utilized for slicing hard materials such as SiC and GaN for power device and high-brightness LED applications.
0.12
Acknowledgement 0.09
0.04 0.00 0.0
formulate procedures to reduce the wire performance transition zone. Under a condition of 18 m/wafer, the average sori value achieved was 16 mm, and the maximum value was 26 mm.
0.06
12
16
20
24
wire consumption (m/wafer)
This research was supported by WPM (World Premium Materials) program funded by the Ministry of Knowledge and Economy of Korea (2010-234-10037886).
Fig. 9. Measurement results for cutting friction vs. feed force as a function of wire consumption per wafer and their COF values.
References friction forces increase until the wires reach the maximum ingot diameter. Then, the forces decrease as the diameter decreases. The friction force on a single wire varies from 0.04 to 0.23 N, depending on the feed position. The main reason for the change in the friction force seems to be the change in the wire–ingot contact area, which is determined by deflection of wire and cutting shape. The coefficient of friction (COF) was around 0.1–0.13. It was slightly higher under a strong wear condition at 12.3 m/wafer, and the minimum value was found under the 24 m/wafer condition. The most critical concern in the MWS process is the flatness of the wafer. Fig. 10 shows the flatness change under different wire consumption conditions, and the sori values are depicted in three graphs. As is clearly seen from the results, the amount of wire consumption per wafer has a close relationship with the flatness trend in each location of the ingot. It especially affects the flatness of wafers near the front of an ingot, where new wire is supplied. These results showed that the transition zone can be reduced by changing the wire consumption conditions, and it is necessary to
[1] Akselrod M, Bruni F (2012) Modern Trends in Crystal Growth and New Applications of Sapphire. Journal of Crystal Growth 360:134–145. [2] Ponce F, Bour D (1997) Nitride-Based Semiconductors for Blue and Green LightEmitting Devices. Nature 386:351–359. [3] Nakamura S, Senoh M, Iwasa N, Nagahama S (1995) High-Brightness InGaN Blue, Green and Yellow Light-Emitting Diodes with Quantum Well Structures. Japanese Journal of Applied Physics 34:L797–L799. [4] Hardin G, Qu J, Shih A (2004) Fixed Abrasive Diamond Wire Saw Slicing of Single-Crystal Cilicon Carbide Wafers. Materials and Manufacturing Processes 19:355–367. [5] Watanabe N, Kondo Y, Ide D, Matsuki T, Takato H, Sakata I (2010) Characterization of Polycrystalline Silicon Wafers for Solar Cells Sliced with Novel FixedAbrasive Wire. Progress in Photovoltaics Research and Applications 18:485–490. [6] Webster J, Tricard M (2004) Innovations in Abrasive Products for Precision Grinding. CIRP Annals 53:597–617. [7] Chiba Y, Tani Y, Enomoto T, Sato H (2003) Development of a High-Speed Manufacturing Method for Electroplated Diamond Wire Tools. CIRP Annals 52(1):281–284. [8] Marinescu I, Rowe W, Dimitrov B, Inasaki I (2004) Tribology of Abrasive Machining Process, William Andrew Publishing: 67–85. [9] Natsu W, Ito Y, Kunieda M, Naoi K, Iguchi N (2005) Effects of Support Method and Mechanical Property of 300 mm Silicon Wafer on Sori Measurement. Precision Engineering 29:19–26.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Multi-wire sawing of sapphire crystals with reciprocating motion of electroplated diamond wires Hyoungjae Kim b, Doyeon Kim b, Chulmin Kim b, Haedo Jeong (2)a,* a b
Graduate School of Mechanical Engineering, Pusan National University, Republic of Korea Net Shape Research Group, Korea Institute of Industrial Technology, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting tool Wafer Multi-wire sawing
This study examined the multi-wire sawing of C-plane sapphire ingots using diamond wires. Feeding new wire during the reciprocating motion of the wire was found to vary the cutting force, wafer shape, and roughness as a result of the break-in effect. The break-in and wire wear seemed to cause a gradual change in the cutting performance along the ingot position. The cutting force results indicated that an inappropriate supply of wire yielded an unbalanced force between the front and back sides of the ingot, which was caused by a difference in the cutting depth along the ingot. The results showed that controlling the wire consumption resulted in an average flatness of 16 mm, with a maximum value of 26 mm. ß 2013 CIRP.
1. Introduction Sapphire, GaN, and SiC are representative single crystals that have been increasingly used for LED and power device applications [1–3]. It is well known that these crystals are difficult-to-machine materials because of their extreme hardness and the limited selection of tool materials [4]. Because of the increasing use of these materials, a multi-wire sawing (MWS) process is essential for the mass production of wafers with precise shapes by cutting the grown ingots into multiple sliced wafers. The MWS process is the most critical process to determine the final shape accuracy of a wafer. The conventional MWS process uses SiC abrasives suspended in a glycol-based lubricant [5]. However, sawing with a SiC slurry generates a large amount of waste and is ineffective for cutting hard crystals. For higher material removal efficiency and the reduction of waste, diamond wire has been adopted for cutting sapphire, SiC, and GaN materials [6,7]. However, precision shape control is still a problem because of the hardness, tool wear, and excessive wire deflection of diamond wire caused by inappropriate cutting conditions. Although diamond wire has been around for a long time, its application to the wafering process is relatively new, and its machinability has never required such a high level of shape accuracy for high-quality wafer production. During the wire sawing of sapphire ingots, wires move back and forth to provide better cutting performance through a higher material removal rate and more effective use of the diamond wires. In addition, as shown in Fig. 1, the wire or ingot is rocked during the cutting process to improve the removal rate by reducing the contact area between the ingot and the wires. The reciprocating motion of the wire has to be optimized by compensating for the wear of the diamond abrasives. Moreover, the feeding speed profile
* Corresponding author. 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.122
Fig. 1. Schematic view of wire sawing of sapphire wafer using diamond wire (left) and optical microscopic image of diamond wires with diameter of 180 mm and diamond size of 30–40 mm (right).
of an ingot into a diamond-coated wire must be considered to continuously compensate for the changing intersection areas of the sapphire ingot. In this study, the authors examined the effect of diamond-wire wear on various cutting performance variables of a C-plane sapphire, such as the roughness, thickness variation, and flatness. A tool dynamometer was used to measure the variation in the vertical feeding force, as well as the lateral cutting force, to better understand the material removal phenomenon in sapphire. 2. Wire motion and cutting profile 2.1. Reciprocation motion of wire In a conventional wire cutting process, uncoated piano wire runs one way at high speed in the presence of a uniformly distributed SiC slurry. Therefore, in the case of bare-wire MWS, the cutting abilities of the multiple wires are approximately the same over the multiple contact positions because of the evenly distributed cutting abrasives. However, when using diamondcoated wires in the MWS process, the wire repeatedly moves back and forth at high speed while progressively advancing by a given amount, defined as v0(tcf tcb) (Fig. 2). Therefore, the cutting
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
336
Fig. 2. Reciprocating motion of diamond wire during MWS process. tcy is the cycle time, and tcf and tcb are the times for the constant feed forward and backward, respectively. Fig. 3. Experimental setup for cutting force measurement.
ability of the diamond-coated wire degrades along the wire position because the sharp edges of the diamond abrasives are gradually worn down by repeated contacts. Thus, it is important to predict the total number of contacts at a fixed location for the wire. Therefore, the cutting ability of diamond coated wire degrades along the wire position since the sharp edges of diamond abrasives gradually worn by repeated contacts. So, it is important to predict the total number of contact at fixed location of a wire.
dynamometer had sensors embedded in four different locations, which could be used to obtain spatial information about the cutting force. Through the dynamometer measurements, the forces at the front and backside positions of the ingot could be analyzed to compare the cutting performances of new and used wires.
4. Results and discussion
2.2. Feed speed of ingot
4.1. Effect of wire break-in and wear on cutting performance
The ingot feed speed is a critical factor to achieve accurate wafer shape characteristics such as the flatness and total thickness variation (TTV). The material removal rate is inversely proportional to the cross-sectional area. Thus, the feed speed profile for a circular ingot can be simply calculated based on its diameter change according to the feeding direction. Therefore, the feed speed used in this experiment can be expressed as a function of wafer radius R and proportional constant k, as written in Eq. (1).
Diamond abrasives are electroplated on a wire using a nickel layer. Although this nickel layer ensures that the diamond abrasives are tightly secured to the wire surface, the layer on top of the abrasives suppresses the cutting performance because it prevents the diamonds from contacting the workpiece. Therefore, this layer should be properly dressed or broken-in to ensure higher cutting performance. An experiment that involved cutting a 4-in sapphire was performed to determine the effect of the break-in on the cutting performance, using a wire consumption of 106 m/wafer and a wire speed and tension of 400 m/min and 35 N, respectively. Fig. 4 shows the thickness variation at different wafer locations. The results clearly show that the thickness of the wafer increased as the wafer number increased. This increase in wafer thickness implies that the kerf loss decreased as a result of a decrease in the equivalent diameter of the cutting edges in the wire.
k VðzÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 R2 ðR zÞ2
(1)
3. Experimental setup 3.1. Multi-wire saw setup
855
average thickness ( m)
Experiments were performed using a production-scale multiwire saw machine, which had a maximum wire speed and tension of 700 m/min and 40 N, respectively. The diamond wire used in the experiment had a core diameter of 0.18 mm, and the attached diamond size was 30–40 mm. An experiment to investigate the break-in effect was performed by supplying a large amount of new wire to clearly see the tool wear transition. In addition, the wire consumption was varied from 24 to 12.3 m/wafer to determine the effect of the wire consumption on the cutting force deviation, as shown in Table 1.
850 845 840 835 830
Table 1 Experimental conditions.
0
Parameters
Conditions
Ingot Wire saw Main roller distance Wire speed Wire tension Wire consumption Cutting fluid Wire
4 in. single crystal sapphire, C-plane 470 mm 400, 500, 700 m/min 35, 40 N 106, 24, 18, 12.3 m/wafer 5 wt% concentration diluted with tap water D0.18 mm piano steel, 30–40 mm diamond
3.2. Tool dynamometer setup During the experiment, an ingot was attached to a dynamometer using an epoxy-based wax. A piezoelectric sensor was used to measure the dynamic cutting force. As shown in Fig. 3, the
10
20
30
40
50
60
wafer No. Fig. 4. Wafer thickness variation across ingot position.
It should be noted that in the initial 5–6 wafers, the thickness rapidly decreased with increasing wafer number, after which the thickness increased with the wafer number. This phenomenon is frequently observed in the diamond wire sawing process, and those initial wafers are usually related to the yield loss. It seems that this phenomenon involves two causes. The first is the break-in effect for large abrasives composed of large diamonds or agglomerated abrasives on a wire. In this stage, the highest spots of the covered nickel layer are rapidly removed and the sharp edges of the diamonds are exposed to increase the cutting performance by repeated contact with the sapphire.
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
Therefore, the high spots on a wire will remove a larger quantity of material around the front side of the ingot, which produces thin wafers near the feeding area. Spots that are higher than the average height are then rapidly worn down, and the diametric deviation is rapidly stabilized by the concentrated wear of those abrasives. The other phenomenon is the gradual wear of the diamond abrasives, which reduces the diameter of the wire and causes the wafer thickness to increase, with less kerf loss. Fig. 5 shows the roughness variation trend across the wafer locations. The roughness is generally determined by the stochastic nature of wire sawing and is a function of the indentation depth and feed speed [8]. As shown in Fig. 5, the roughness gradually decreases with an increase in the wafer number. Considering the results from Fig. 4, the enhanced roughness near the backside of the ingot is clearly the result of the wear of the diamond wire. 900
Roughness, Ra (nm)
750
600
450
337
4.2. Effect of wire consumption on cutting force Fig. 7 shows the measurement results for the ingot feed force against a wire as a function of the wire deflection and given wire tension. The feed forces were measured using a dynamometer by feeding an ingot into stationary diamond wires. Considering the wire deflection and ingot position, the theoretical feed force f can be calculated as follows in Eq. (2).
d
f ¼ 2ðT 0 þ DTÞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L2 =4 þ d
(2)
where T0 is the given wire tension and DT is the additional tension from the elastic elongation of the wire caused by wire deflection. d and L are the wire deflection and wire length between the rollers, respectively. As shown in Fig. 7, the measured feed forces are slightly higher than the theoretically predicted one, which means that the tension of the wire wound inside the main rollers is higher than the controlled value. This increased tension is caused by the rotational friction and inertia of the guide rollers located between the tension control dancer and the main roller. The results showed that the feed force is a linear function of the tension and deflection. Therefore, if the feed force can be measured during cutting, the inprocess deflection of the wire can be evaluated.
300
2.0
25N_Expe 30N_Expe 35N_Expe 40N_Expe 25N_Pred 30N_Pred 35N_Pred 40N_Pred
1.8 150
1.6 1.4
5 10 15 20 25 30 35 40 45 50 55 60 65 Wafer No.
Fig. 5. Roughness variation across ingot positions caused by degradation of diamond abrasives, along with average values at five measurement points.
Feed force (N)
-5 0
1.2 1.0 0.8 0.6 0.4 0.2
The graph in Fig. 6 shows the sori value, and the picture above the graph depicts the actual wafer shape after the MWS process. The ‘‘sori’’ of the wafer is defined as the difference between the maximum and minimum distances to the front surface of a free, unclamped wafer from the least-squares reference plane of the front surface [9]. The wafers near the wire supply position show high sori values, and the value decreases as the wafer number increases. In addition, several wafers near the feed-in position show irregular shapes, which seem to be caused by the break-in effect. The high sori values near the inlet position seem to be caused by an unstable wire condition, where the surface condition of the wire rapidly changes as a result of the break-in of the nickel layer and transient wear of the high cutting edges. Therefore, the wire consumption and tool life should be properly selected for precise cutting results by minimizing the transition wear of the diamond wire.
Fig. 6. Wafer shapes after cutting process and sori values.
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Based on these results, the feed force was measured during the cutting of 4-in. sapphire ingots under various wire consumption conditions. Fig. 8(a)–(c) show the effect of the wire consumption on the feed force trend. As the ingot feeds into the wires, the feed force is gradually increased with an increase in the cross section of the ingot. Then, the force is reduced with a reduction in the diameter. However, the maximum force occurs after 50–70% of the feed position, and the maximum point moves back as the wire consumption is reduced. This result indicates that the wire deflection increases as the wire consumption decreases. The wire deflection is accumulated as a form of elastic energy in the wires until the ingot passes the maximum diameter. This accumulated elastic energy in the wires exerts a high contact force on the ingot. Therefore, faster material removal occurs in the last half of the ingot from the combination of the accumulated elastic energy in wire and the rapid reduction in the cross-sectional area of the ingot. Thus, the force curve rapidly drops after its maximum. In addition, Fig. 8(d) shows the ratio of the feed forces measured by the sensors located at the front and back sides of the ingot. These results indicate that the penetration depth of the wire into the ingot is different between the front and the back sides and increases when the wire consumption is smaller than a critical value. In this case, a nonuniform cutting depth seems to affect the cutting accuracy because in the case of a 12.3-m/wafer condition, the sori value was measured at over 100 mm. Fig. 9 shows the friction force trend during wire sawing under different wire consumption conditions. As shown in the graph, the
H. Kim et al. / CIRP Annals - Manufacturing Technology 62 (2013) 335–338
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5. Conclusion This study examined the multi-wire sawing of single-crystal sapphire ingots to determine the effects of the wire conditions on the cutting results. The results showed that the break-in of a wire significantly affects the shape quality of the cut wafers. In particular, the initial wear of an electroplated diamond wire has a strong relationship with the quality of the wafers located near the wire inlet position. In addition, the flatness of the wafer can be controlled by controlling the wire consumption per wafer through minimizing the break-in zone and excessive wear. The results of this study are not only applicable to the precision sawing of sapphire ingots but can also be utilized for slicing hard materials such as SiC and GaN for power device and high-brightness LED applications.
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formulate procedures to reduce the wire performance transition zone. Under a condition of 18 m/wafer, the average sori value achieved was 16 mm, and the maximum value was 26 mm.
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This research was supported by WPM (World Premium Materials) program funded by the Ministry of Knowledge and Economy of Korea (2010-234-10037886).
Fig. 9. Measurement results for cutting friction vs. feed force as a function of wire consumption per wafer and their COF values.
References friction forces increase until the wires reach the maximum ingot diameter. Then, the forces decrease as the diameter decreases. The friction force on a single wire varies from 0.04 to 0.23 N, depending on the feed position. The main reason for the change in the friction force seems to be the change in the wire–ingot contact area, which is determined by deflection of wire and cutting shape. The coefficient of friction (COF) was around 0.1–0.13. It was slightly higher under a strong wear condition at 12.3 m/wafer, and the minimum value was found under the 24 m/wafer condition. The most critical concern in the MWS process is the flatness of the wafer. Fig. 10 shows the flatness change under different wire consumption conditions, and the sori values are depicted in three graphs. As is clearly seen from the results, the amount of wire consumption per wafer has a close relationship with the flatness trend in each location of the ingot. It especially affects the flatness of wafers near the front of an ingot, where new wire is supplied. These results showed that the transition zone can be reduced by changing the wire consumption conditions, and it is necessary to
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