Materials and Manufacturing Processes ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lmmp20 Developed diamond wire sawing technique with high slicing ability for multicrystalline silicon wafers Ting-Chun Wang , Tsung-Han Yeh , Shao-Yu Chu , Hsin-Ying Lee & Ching-Ting Lee To cite this article: Ting-Chun Wang , Tsung-Han Yeh , Shao-Yu Chu , Hsin-Ying Lee & Ching-Ting Lee (2020): Developed diamond wire sawing technique with high slicing ability for multicrystalline silicon wafers, Materials and Manufacturing Processes, DOI: 10.1080/10426914.2020.1802037 To link to this article: https://doi.org/10.1080/10426914.2020.1802037 Published online: 13 Aug 2020. Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=lmmp20 MATERIALS AND MANUFACTURING PROCESSES https://doi.org/10.1080/10426914.2020.1802037 Developed diamond wire sawing technique with high slicing ability for multicrystalline silicon wafers Ting-Chun Wanga, Tsung-Han Yeha, Shao-Yu Chua, Hsin-Ying Lee a , and Ching-Ting Leea,b a Department of Photonics, National Cheng Kung University, Tainan, Taiwan, Republic of China; bDepartment of Electrical Engineering, Yuan Ze University, Taoyuan, Taiwan, Republic of China ABSTRACT ARTICLE HISTORY In this work, various reciprocating cycle times of 80, 160, 240, and 320 sec in the diamond wire sawing (DWS) process were adjusted to improve the slicing ability in solar industry. During the same slicing time, the long reciprocating cycle time had less velocity inverse points in comparison with the short reciprocating cycle time. Consequently, the total friction force of the slicing wires used in the DWS process with the short reciprocating cycle time was larger than that of the slicing wires used in the DWS process with the long reciprocating cycle time. It was noting that the lower diamond consumption and better slicing ability in the DWS process with a reciprocating cycle time of 320 sec was obtained in comparison with a reciprocating cycle time of 80 sec. However, since the diamond grits with too high slicing strength to collide the Si material, the serious damages were form on the wafer edge. Therefore, the edge chipping increased to 1.63% as the reciprocating cycle time of 320 sec. The highest mass production yield of 94.22% and the lowest edge chipping of 1.23% for the DWS-sliced mc-Si wafers were obtained as the suitable reciprocating cycle time was 240 sec. Received 4 March 2020 Accepted 6 July 2020 Introduction According to the report from researching group around the world, the cost of the sliced silicon (Si) wafers accounts for about 40% of the total cost of the solar cell fabrication in photovoltaic industry. For solar corporations, in order to reduce the fabricated cost and keep the competitiveness in industry, the best solution is to improve the performance and increase the mass production yield of the sliced multicrystalline silicon (mc-Si) wafers. From 1980s, multi wire slurry (MWS) slicing technique with silicon carbide (SiC) and polyethylene glycol (PEG) slurry abrasives has been skillfully used to slice the mc-Si wafer in solar cell industry.[1–3] In recent years, diamond wire sawing (DWS) technique has rapidly gained solar cell industrial attention owing to it has some inherent advantages, such as less consumption of Si per unit capacity, Si kerf-recycling, and short time of slicing process.[4,5] In DWS technique, the diamond abrasive grits are attached on a core steel wire by electroplating method. By using the highest hardness diamond grits and unique reciprocating sliced mode, DWS technique provides stronger wire axial cutting force and radial cutting force to slice the Si materials.[6–8] Consequently, DWS technique has higher productivity than the traditional MWS slicing technique.[9] However, the yield of the DWS-sliced mc-Si wafers is lower than that of the MWSsliced mc-Si wafers in per slicing process. Besides, since the diamond wires and unique slicing coolants for cooling are expensive, the cost of the DWS-sliced mc-Si wafers is higher than that of the MWS-sliced mc-Si wafers. Consequently, how to enhance the sliced ability for the mc-Si wafers and limit the additional cost have become a serious issue in DWS technique. CONTACT Hsin-Ying Lee © 2020 Taylor & Francis hylee@ee.ncku.edu.tw KEYWORDS Slice; cutting; saw; sawing; cycle; optimization; photovoltiacs; diamond; reciprocating; machinability; abrasion The high quality wafers are achieved by improving the slicing ability of DWS technique. The slicing ability is a comprehensive index of all sawing related parameters,[10] including work-piece feed rate, slicing wire speed and tension, coolant temperature. To enhance the sliced ability of DWS technique, several methods, such as larger diamond grits size, high diamond density, coolant refresh on time, and equipment hardware retrofit, were used.[11–17] Unfortunately, using these methods would generate the additional cost. However, the reduction cost in DWS system is the highest priority for maintaining the competitive advantage of the solar cell industry. Consequently, in this work, in order to avoid additional costs and achieve high sliced ability, the various reciprocating cycle times in DWS process were designed and investigated for sliced ability for keeping economic benefit of solar industry. Using a longer reciprocating cycle time reduced the alternating frequency between the acceleration and deceleration of the slice wires, which allowed the slice wire to maintain maximum speed for a long time. Moreover, the amounts of velocity reverse point were reduced as the wire cycle path increasing, which could reduce the consumption of the diamond grits mounted and afford larger kinetic energy and momentum on the slicing wire in a long reciprocating cycle time mode. Materials and methods In this work, the mc-Si bricks with dimension of 156 mm × 156 mm × 250 mm were sliced to form 190-μm-thick mc-Si wafers by using a DWS slicing system (Meyer Burger DS264 slicing system). The diamond grits were mounted on the Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China 2 T.-C. WANG ET AL. stainless steel wires by electroplating method. The tension, velocity, and acceleration of the slicing wire were respectively set and fixed to 25 N, 15 m/sec, and 4 m/sec2. The coolant temperature and the total slice process time were 15°C and 180 min, respectively. Various reciprocating cycle times of 80, 160, 240, and 320 sec were designed in DWS slicing process. A batch DWS-sliced mc-Si wafer was 3000 pieces in per slicing process. To analyze the slicing ability of DWS technique, the density and the average height of the diamond grits mounted on the slicing wires was inspected according to the morphology of the diamond wire measured by a scanning electron microscope (SEM). The quality of the DWS-sliced mc-Si wafers was also an important judgment of the slicing ability for DWS technique. Consequently, the total thickness variation (TTV), wafer to wafer thickness aberration (WWTA), saw mark (SM), edge chipping, and yield of the total DWS-sliced mc-Si wafers were directly inspected and judged by using a built-in multifunction optical system in the Meyer Burger Hennecke HE-WI -06s Systems Results and discussion In this work, the cutting motion mode with reciprocating cycle time of 80 sec and 320 sec between the slicing time of 320 sec in DWS slicing process was shown in Fig. 1. In DWS slicing process, the slicing wires were repeatedly pulled forward and pulled-backward slicing wires.[18] Consequently, a partial slicing wire was eliminated after a reciprocating cycle, which was the definition of wire usage consumable in wire saw process. For the reciprocating cycle time of 80 sec, the length of the pulled-forward and the pulled-backward slicing wires was 600.0 m (P1 area in Fig. 1) and 487.5 m(P2 area in Fig. 1), respectively. For the reciprocating cycle time of 320 sec, the length of the pulled-forward and the pulled-backward slicing wires was 2400.0 m (P3 area in Fig. 1) and 2287.5 m (P4 area in Fig. 1), respectively. To compare two reciprocating cycle time conditions during a fixed time period 320 sec, the wire usage consumable of cycle time of 80 sec had achieved to 450.0 m, but wire usage consumable of cycle time of 320 sec was 112.5 m only. It was noting that the usage amount of the slicing wire for long reciprocating cycle time was more economical than that for short reciprocating cycle time. According to the classic physical friction theory,[19] the moveable behavior of a static diamond grit at inverse point position was shown in Fig. 2. The diamond grit was still stationary until the time at t1. Although the diamond grit moved almost from a standstill, it remained stationary at this time (t1). The friction force of the diamond grit was belonged to static friction force (fS) before t1 and the maximum static friction force (fSmax) would be arrived before diamond grit starting to move. Subsequently, when the diamond grit moved (velocity>0 m/sec), the friction force of the diamond grit was converted into the dynamic friction force (fK). According to the classic physical friction theory, the maximum static friction force was larger than the dynamic friction force (fSmax>fK).[20] The larger friction force would let the diamond grits mounted on the slicing wires easily depleted and fell off. As shown in Fig. 1, it could be obviously found that there were 8 and 2 velocity inverse points (velocity = 0 m/sec) as the reciprocating cycle time of 80 sec and 320 sec between the slicing time of 320 sec, respectively. Consequently, it was found that the diamond grits had lower total friction force as the reciprocating cycle time was 320 sec, which could slow down the consumption of the diamond grit and improve the slicing ability in DWS slicing process. The SEM measurement was carried out to observer the morphology of unused diamond wires and the diamond wires used in DWS slicing process with various reciprocating cycle times of 80, 160, 240, and 320 sec after 180 min of slicing process and was shown in Fig. 3(a-e). All elements of diamond wire surface could be directly measured by analysis software in SEM system, including the diamond grit height and wire width of 120 μm. The average diamond height of the unused and used diamond wires was shown in Fig. 4. The average diamond height of the unused wires was 10.35 μm. After 180 min of slicing process, the average diamond height was decrease to 6.25 μm and 5.12 μm as the reciprocating cycle time was 320 sec and 80 sec, respectively. In the short cycle time, more speed inverse points were existed to cause the diamond Figure 1. The cutting motion mode with reciprocating cycle time of 80 sec and 320 sec between the slicing time of 320 sec in DWS slicing process. MATERIALS AND MANUFACTURING PROCESSES 3 Figure 2. The moveable behavior of a static diamond grit at inverse point position. Figure 3. SEM images of (a) unused diamond wires and diamond wires used in DWS slicing process with various reciprocating cycle times of (b) 80, (c) 160, (d) 240, and (e) 320 sec after 180 min of slicing process. grinding compound was worn under high external force. According to the classic physical friction theory, the diamond wire used in DWS slicing process with the short reciprocating cycle time had larger total friction force, which increased the consumption of the diamond grits. The diamond density of the unused wires was 938.7 EA/mm2 which was calculated from a number of diamond grits and surface area. The diamond density of the used wires in DWS slicing process with various reciprocating cycle times of 80, 160, 240, and 320 sec was 502.8, 636.6, 720.4, and 770.7 EA/mm2, respectively. This phenomenon was attributed to that the total friction force of the diamond grits as the reciprocating cycle time of 80 sec was larger than that of the diamond grits as the reciprocating cycle time of 320 sec between the same slicing time, which let the diamond grits mounted on the slicing wires easily depleted and fell off. The wafer quality was also very important issue to evaluate the quality in DWS slicing process, including total thickness variation (TTV), wafer to wafer thickness aberration (WWTA), saw mark Table 1. The Performances of the DWS-sliced mc-Si wafers sliced using various reciprocating cycle times. Cycle time (sec) TTV (%) WWTA (%) SM (%) Edge chipping (%) Yield (%) 80 2.23% 0.56% 2.14% 1.88% 92.22% 160 1.38% 0.52% 2.12% 1.62% 93.46% 240 0.81% 0.42% 1.33% 1.23% 94.22% 320 1.14% 0.45% 1.53% 1.63% 93.88% 4 T.-C. WANG ET AL. Figure 4. The average diamond height of the unused diamond wires and diamond wires used in DWS slicing process with various reciprocating cycle times after 180 min of slicing process. (SM), edge chipping. The above characteristics would affect the overall performance of the mc-Si solar cells.[21,22] In this work, the TTV, WWTA, SM, edge chipping, and yield of the DWS-sliced wafers were directly inspected and judged using a built-in multifunction optical system in the Meyer Burger Hennecke system and were listed in Table 1. The TTV, WWTA, SM, and edge chipping wafers were decreased with the reciprocating cycle time increased from 80 sec to 240 sec. The lowest TTV, WWTA, SM, and edge chipping of the DWS wafers sliced using fixed reciprocating cycle time of 240 sec were 0.81%, 0.42%, 1.33%, and 1.23%, respectively. However, the TTV, WWTA, SM, and edge chipping increased by further increasing the reciprocating cycle time to 320 sec. As shown in Fig. 4, although the best slicing ability (highest diamond height and largest diamond density) in DWS process was achieved as the reciprocating cycle time of 320 sec. However, the edge chipping increased to 1.63% as the reciprocating cycle time of 320 sec. This phenomenon was attributed to that the diamond grits with too large slicing strength to collide the Si material,[23,24] which caused the serious damage on the wafer edge. Consequently, the yield of the DWS-sliced mc-Si wafers was decrease from 94.22% to 93.88% by further increasing the reciprocating cycle time from 240 sec to 320 sec. Fortunately, to reduce the defect distribution and to improve the yield for the DWS-sliced mc-Si wafers could be simultaneously achieved by setting the suitable reciprocating cycle time. By adjusting the parameter recipe of the DWS system, every diamond grit could be effectively used for cutting process, which did not generate the additional cost. In the future, the thinner slicing wires mounted the smaller diamond grits will be considered for minimizing consumption of Si per unit capacity in wafer-sliced industry. Generally, the thinner slicing wires were easy fracture under the high wire tension. On other words, the limited boundary condition in slicing process will be a serious issue to enhance sliced ability. To obtain high quality wafers by using the DWS system with slicing wires mounted the smaller diamond grits, the limitation of wire acceleration and wire tension was a big challenge. Consequently, the long cycle-time slicing mode in DWS process can enhance the sliced ability for high performance wafers. Acknowledgments This work was supported by the Ministry of Science and Technology of the Republic of China under contract No. MOST 107-2221-E-006-144 and MOST 108-2221-E-006-196-MY3. 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