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An experimental investigation into soft-pad grinding of wire-sawn silicon wafers
International Journal of Machine Tools & Manufacture 44 (2004) 299–306 www.elsevier.com/locate/ijmatool
An experimental investigation into soft-pad grinding of wire-sawn silicon wafers Z.J. Pei a,, S. Kassir b, Milind Bhagavat c, Graham R. Fisher c a
Department of Industrial and Manufacturing Systems Engineering, Kansas State University, 237 Durland Hall, Manhattan, KS 66506-5101, USA b Strasbaugh, Inc., 825 Buckley Road, San Luis Obispo, CA 93401, USA c MEMC Electronic Materials, Inc., 501 Pearl Drive, St. Peters, MO 63376, USA Received 11 August 2003; received in revised form 8 September 2003; accepted 16 September 2003
Abstract Silicon is the primary semiconductor material used to fabricate microchips. A series of processes are required to manufacture high-quality silicon wafers. Surface grinding is one of the processes used to flatten wire-sawn wafers. A major issue in grinding of wire-sawn wafers is reduction and elimination of wire-sawing induced waviness. Results of finite element analysis have shown that soft-pad grinding is very effective in reducing the waviness. This paper presents an experimental investigation into soft-pad grinding of wire-sawn silicon wafers. Wire-sawn wafers from a same silicon ingot were used for the study to ensure that these wafers have similar waviness. These wafers were ground using two different soft pads. As a comparison, some wafers were also ground on a rigid chuck. Effectiveness of soft-pad grinding in removing waviness has been clearly demonstrated. # 2003 Elsevier Ltd. All rights reserved. Keywords: Grinding; Lapping; Machining; Material removal; Semiconductor material; Silicon wafer; Slicing
1. Introduction 1.1. Manufacturing processes for silicon wafers Semiconductors are widely used in every type of microelectronic applications, including computer systems, telecommunications equipment, automobiles, consumer electronics, industrial automation and control systems, and analytical and defense systems. The majority of semiconductors are built on silicon wafers [1]. About 150 million silicon wafers of different sizes are manufactured each year worldwide [2]. In 2000, worldwide revenue generated by silicon wafers was US$ 7.5 billion [3]. Semiconductor devices built on these wafers generated US$ 200 billion in revenues [4]. To manufacture high-quality silicon wafers, a sequence of processes is required. As shown in Fig. 1, a typical manufacturing process flow includes the following major processes [5–11]. Note that some processes Corresponding author. Tel.: +1-785-532-3436; fax: +1-785-5323738. E-mail address: [email protected] (Z.J. Pei).
0890-6955/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2003.09.006
are omitted in Fig. 1 for simplicity, for example, edge grinding, edge polishing, and laser marking. 1. Crystal growth—to produce crystal ingots; 2. Slicing (ID sawing or wire sawing)—to slice the ingots into wafers of a thin disk shape; 3. Flattening (lapping or grinding)—to flatten the wafer surface; 4. Etching—to chemically remove processing-induced damage without introducing further mechanical damage; 5. Polishing—to obtain a mirror surface; and 6. Cleaning—to remove the polishing agent or dust particles. 1.2. Pros and cons of wire sawing Until recently, internal-diameter (ID) sawing had been the dominant slicing method for the past three decades [12,13]. Wire sawing, however, is now fully established as the preferred method of slicing large diameter ingots. Although the time per cut in wire sawing is longer than that in ID sawing, the overall throughput
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the ingot, providing a flat reference plane for subsequent processes [17]. 1.3. Pros and cons of grinding Conventionally, wafers after slicing will go through lapping operations for flattening. In a double side lapping operation, batch of wafers (for example, 20 wafers) are manually loaded into a lapping machine. Aluminum-oxide slurry is injected between two metal plates that rotate in opposite directions [18,19]. Despite its widespread use, lapping has the following drawbacks: labor intensive, expensive consumable (slurry), and time consuming. Grinding can be used for flattening sliced wafers to replace or partially replace lapping [11,14,20–23]. The advantages of grinding over lapping include:
Fig. 1. Typical process flow for manufacturing silicon wafers.
of wire sawing is superior because wire sawing can slice hundreds of wafers per cut, while ID sawing cuts one wafer at a time. Furthermore, owing to thinner kerf losses, wire-sawing yields more wafers per unit length of crystal ingot than ID sawing. An undesirable phenomenon associated with wire sawing is the waviness induced by the sawing process. The waviness is also called long cycle swelling or unevenness [14], or wavy stripes [15]. Typically, the wavelength ranges from 0.5 to 30 mm [14]. The generating mechanism of this waviness is not fully understood yet. This has been the main reason that it is very difficult to eliminate waviness at wiresawing operation. If subsequent processes do not remove the waviness, it will adversely affect the flatness, especially the site flatness, of the wafers. A frequently used parameter to measure site flatness is SBIR (site flatness, back reference surface, ideal reference plane, range) [16]. It is the sum of the maximum positive and negative deviations of the surface in a certain area of the wafer from a theoretical reference plane that is approximately parallel to the back surface of the wafer and intersects the front surface at the center of the area. Typical size of the area is 20 20 mm2 for ordinary wafers and 30 35 mm2 for advanced applications. Waviness has not been a problem for ID sawing. With modern ID sawing technology, the backside of the wafer is ground before the wafer is sliced off from
. Grinding is fully automatic. In lapping, wafer loading and unloading is done manually, not only increasing labor costs, but also causing frequent wafer breakage. . Grinding uses fixed-abrasive wheels instead of loose abrasive slurry, hence the cost of consumables per wafer is lower. Furthermore, fixed-abrasive grinding wheels are more benign to the environment than lapping slurry. . Grinding cuts one wafer at a time instead of batch processing. In batch operations, the work in process (WIP) inventory is higher; and once problems occur in production, more defects will be produced before the problems are fixed. Furthermore, a thicknesssorting step is often needed before lapping to ensure that only wafers with small wafer-to-wafer thickness variation are put on the same lapping machine. Often times, as there are not enough production wafers to fill in all the lapping carriers on one machine, extra dummy wafers have to be used, hence adding more cost. . Grinding has higher throughput. For example, a typical lapping operation will take about 40 min to reduce the wafer thickness (200 or 300 mm in diameter) by about 80 lm, while it takes about 30–60 s for grinding to reduce the wafer thickness (200 or 300 mm in diameter) by about 45–75 lm [11]. Grinding has its drawbacks too. It has been reported that lapping can effectively remove waviness but grinding cannot [14,15,24]. Differences in waviness removal by grinding and lapping have been investigated using finite element analysis (FEA) [24]. The results have revealed that, under the same applied force, the relative peak displacement (the displacement of waviness peaks relative to the waviness valleys) at the deformed state in lapping is much smaller, only 1/55 to 1/36 of that in grinding. Smaller relative peak displacement is desir-
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Fig. 3.
Fig. 2. Deformation of a wafer during rigid-chuck grinding and return to its original shape.
able for reducing or eliminating waviness. If a wavy wafer deforms elastically during machining (grinding or lapping), after the operation it will spring back to its original shape thus preserving the waviness. This is illustrated in Fig. 2. 1.4. Approaches to waviness reduction in wafer grinding Wafer grinding is potentially a more cost-effective process than lapping for flattening wire-sawn silicon wafers. However, its success in replacing lapping depends critically on whether the wire-sawing induced waviness can be removed. Several techniques to remove or reduce waviness have been proposed, including wafer grinding followed by lapping [11], reduced chuck vacuum [25], use of ‘‘soft-pad’’ [26], and wax mounting [14]. FEA modeling has shown that soft-pad grinding is the most promising approach because it is effective in reducing the waviness and easily implemented [27]. 1.5. Literature review on soft-pad grinding Soft-pad grinding first appeared in public domain in 1999 through an US patent by Kassir and Walsh [26]. When grinding the first side of a wire-sawn wafer, a perforated resilient pad is inserted in between the wafer and the ceramic chuck (a rigid chuck). The soft pad accommodates and supports the wavy surface of the wafer and holds the wafer in an undeformed condition. As a result, the waviness of the top surface is removed effectively by grinding. This ground surface will be the flat reference plane for grinding the other side of the wafer on a conventional ceramic chuck. The process is illustrated in Fig. 3.
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Illustration of soft-pad grinding.
In 2001, both Pei and Fisher [28] and Pietsch and Kerstan [29] briefly reported experimental evidence of soft-pad grinding. However, systematic experimental studies on soft-pad grinding of wire-sawn wafers are not available in the literature. A two-dimensional (2D) FEA study [30] showed that soft-pad grinding is effective in removing waviness. Use of a soft pad in between the wafer and the rigid chuck significantly reduces the elastic deformation of the wafer in comparison with grinding on a rigid chuck. This 2D FEA modeling was used to predict the effects of several parameters (such as Young’s modulus and Poisson’s ratio of the soft pad) on the effectiveness of waviness removal. In a follow-up study [31], a 25 (fivefactor two-level) full factorial design was employed to reveal the main effects as well as the interaction effects of five factors (waviness wavelength of silicon wafers; Young’s modulus, Poisson’s ratio, and thickness of soft pads; and grinding force) on effectiveness of waviness reduction. Major conclusions from the FEA investigations include: 1. Waviness wavelength: the longer the wavelength, the more difficult to remove the waviness. 2. Wafer thickness: the thinner the wafer, the more difficult to remove the waviness. 3. Grinding force: the larger the grinding force, the more difficult to remove the waviness. 4. Soft pad properties: it becomes more difficult to remove the waviness as the pad’s Young’s modulus increases, or as the pad’s Poisson’s ratio increases, or as the pad thickness decreases. 1.6. Outline of this paper This paper, for the first time, presents an experimental investigation into soft-pad grinding of wiresawn silicon wafers. Wire-sawn wafers from a same
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silicon ingot were used for the investigation to ensure that these wafers have similar waviness. These wafers were ground using two different soft pads. As a comparison, some wafers were also ground on a rigid chuck. For each of these conditions (with pad A, with pad B, and without any soft pad), two different process sequences were tested. The experimental results have provided verifications for some earlier predictions through the FEA. There are four sections in this paper. Following this Introduction section, Section 2 describes experimental conditions and procedures. In Section 3, experimental results will be presented and discussed. Finally, conclusions are drawn in Section 4.
2. Experimental conditions and procedures Grinding experiments were conducted on a Strasbaugh grinder of Model 7AF. Fig. 4 illustrates the wafer grinding process. Grinding wheels are diamond cup wheels. The workpiece (wafer) is held on a porous ceramic chuck (a rigid chuck) by means of vacuum for conventional grinding. For soft-pad grinding, a perforated soft pad is inserted in between the wafer and the ceramic chuck. The rotation axis for the grinding wheel is offset by a distance of the wheel radius relative to the rotation axis for the wafer. During grinding, the grinding wheel and the wafer rotate about their own
rotation axes simultaneously, and the wheel is fed towards the wafer along its axis. Grinding wheels used were resin-bonded diamond wheels, with diameter of 280 mm. The grit size for the coarse wheel was mesh #320. The grit size for the fine wheel is mesh #2000. Single crystal silicon wafers of 200 mm in diameter with ð1 0 0Þ planes as major surfaces were used for this investigation. All the test wafers were sliced (by wire sawing) at same time from a same crystal ingot, hence had similar waviness (wavelength and height). Grinding parameters and their values are listed in Table 1. Note that there were three feedrate values, used for three sequent steps, respectively. During grinding, deionized (purified) water was used to cool the grinding wheel and the wafer surface. The coolant was supplied to the inner side of the cup wheel, at a flow rate of 3 gal/min. Two types of soft pads (A and B) were tested. The diameter of these pads was 200 mm. Small holes were perforated in both pads to allow vacuum to go through so that the wafer could be held on the ceramic chuck (with the soft pad in between). Both pads have the same nominal mechanical properties and thickness. Detailed information on the pad properties is proprietary. The major difference between the two pads was the arrangement of the perforated holes. Tables 2 and 3 show two grinding sequences: ‘‘flip once’’ and ‘‘flip twice’’, respectively. One group of wafers were ground following the ‘‘flip once’’ sequence, and another group of wafers were ground following the ‘‘flip twice’’ sequence. Under each grinding condition, two or more wafers were ground. However, only one wafer from each grinding condition was chosen for single-side polishing. Same amount of material (15 lm thick) was polished off from the same side of every wafer, using an identical polishing recipe. These polished wafers were inspected under a magic mirror. Pictures of the polished surfaces were taken by the magic mirror to compare the residual waviness. Information on the magic mirror technology can be found in [32–34] and also at website.
3. Results and discussion The experimental results are summarized in Table 4. They will be discussed together with some analytical results published before. 3.1. Grinding with a soft pad versus without a soft pad
Fig. 4. Illustration of wafer grinding.
Two wafers (#5 and #6 in Table 4) ground without a soft pad clearly exhibit visible waviness. On the
Z.J. Pei et al. / International Journal of Machine Tools & Manufacture 44 (2004) 299–306 Table 1 Grinding parameters and their values Parameter
Unit
Coarse grinding
Fine grinding
Removal Wheel speed Chuck speed Feedrate for step 1 Feedrate for step 2 Feedrate for step 3
lm rev s1 (rpm) rev s1 (rpm) lm s1
20 32.05 (1923) 1.67 (100) 4
17 72.50 (4350) 9.83 (590) 1
lm s1
0.5
0.5
lm s1
0.3
0.3
Table 2 ‘‘Flip once’’ sequence Operation
Description
1
Coarse and fine grinding the first side of the wafer on a soft pad Flip the wafer Coarse and fine grinding the second side of the wafer on a rigid chuck
2 3
Table 3 ‘‘Flip twice’’ sequence Operation
Description
1
Coarse grinding the first side of the wafer on a soft pad Flip the wafer Coarse and fine grinding the second side of the wafer on a rigid chuck Flip the wafer Fine grinding the first side of the wafer on a rigid chuck
2 3 4 5
contrary, all four wafers (#1 to #4 in Table 4) ground with soft pads do not show any waviness. The effectiveness of soft pads in removing wire-sawing induced waviness is clearly demonstrated. This experimental observation has substantiated the simulation results by an earlier FEA study [30]. As shown in Fig. 5, use of a soft pad can significantly reduce the relative peak displacement (the displacement of waviness peaks relative to the waviness valleys, measuring the reduction of the vertical peak-to-valley distance), and hence can effectively reduce waviness. Simulations by the FEA have shown that, under a specific grinding condition, when grinding without a soft pad, the relative peak displacement reached 20 lm (i.e., the waviness peaks touched the ceramic chuck), hence the waviness cannot be removed. Under the same grinding condition, when grinding with a soft pad of certain properties (Young’s modulus ¼ 1 MPa and Poisson’s ratio ¼ 0:1, and pad thickness ¼ 1 mm), the
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relative peak displacement was only 0.1 lm (almost the same amplitude as the original waviness), hence the vast majority of the waviness will be removed (the survived waviness will have an amplitude of about 0.1 lm). 3.2. ‘‘Flip once’’ sequence versus ‘‘flip twice’’ sequence Effects of sequence (‘‘flip once’’ versus ‘‘flip twice’’) are more obvious for grinding without soft pads than with soft pads. It can be seen that more waviness strips are visible on wafer #5 (ground by ‘‘flip once’’) than on wafer #6 (ground by ‘‘flip twice’’). In other words, when the ‘‘flip once’’ sequence was used (wafer #5), more waviness strips survived the grinding operation. This observation substantiates the FEA predictions published earlier in this journal [35,36]. The FEA simulation of wire-sawn wafer grinding on a rigid chuck [35] and on a soft pad [36] has shown that waviness with longer wavelength is more difficult to remove. It has also been pointed out that waviness is more difficult to remove with fine grinding than with coarse grinding, because the grinding force is much lower in coarse grinding of silicon wafers than in fine grinding. The ‘‘flip twice’’ sequence only employed coarse grinding to the first side of the wafer on the soft pad, while in the ‘‘flip once’’ sequence, both coarse grinding and fine grinding were applied to the first side. Therefore, more waviness strips survived the grinding with the ‘‘flip once’’ sequence. This experimental observation supports a practical implication predicted from an FEA study of wire-sawn wafer grinding on a rigid chuck [35]. In that study, it was shown that as grinding force increased, relative peak displacement increased. In other words, it was easier to remove the wire-sawing induced waviness when the grinding force was lower. Normally, the grinding force in coarse grinding was much lower than that in fine grinding. Therefore, it was predicted that the ‘‘flip twice’’ sequence (it only involves coarse grinding to grind the first side of the wafer) should be more effective than the ‘‘flip once’’ sequence (it involves both coarse and fine grinding to grind the first side of the wafer) in removing the wire-sawing induced waviness. Effects of sequence (‘‘flip once’’ versus ‘‘flip twice’’) on waviness removal are hardly visible for grinding with soft pads. All four wafers (#1 to #4 in Table 4) ground with soft pads do not show any waviness. However, these wafers exhibit other non-uniform patterns (‘‘clouds’’ patterns). These ‘‘clouds’’ patterns have been briefly mentioned by Pei and Fisher [28], and Pietsch and Kerstan [29]. They are the reflection of surface undulations. These ‘‘clouds’’ patterns are probably caused by the non-uniformity of the soft pads. It looks like that the ‘‘clouds’’ patterns on the wafers (#2 and #4) ground by the ‘‘flip twice’’ sequence are less severe
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Table 4 Experimental results Flip once
Flip twice
Soft pad A
Wafer #1
Wafer #2
Soft pad B
Wafer #3
Wafer #4
Wafer #5
Wafer #6
No soft pad
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2. From the viewpoint of removing waviness, ‘‘flip twice’’ sequence is better than ‘‘flip once’’ sequence. 3. Both soft pads under this study are effective in removing waviness. ‘‘Clouds’’ patterns appeared on wafers ground on both soft pads, and further investigations into the ‘‘clouds’’ patterns are needed.
Acknowledgements This work was supported in part by the National Science Foundation under grant DMI-0218237 and by the Advanced Manufacturing Institute of Kansas State University. Fig. 5. FEA simulated comparison between soft-pad grinding and rigid-chuck grinding.
References than those on wafers (#1 and #3) ground by the ‘‘flip once’’ sequence. However, this observation is not conclusive (due to limited number of experiments and lack of quantitative measurement) and further investigation is needed. 3.3. Effects of different pads From Table 4, it can be seen that both pads are effective in removing waviness. Furthermore, the ‘‘clouds’’ patterns on the wafers (#1 and #2) ground by pad A are less severe (that is, the ‘‘clouds’’ areas are less different from other areas in term of darkness/ brightness) than those on wafers (#3 and #4) ground by pad B. Again, this observation is not conclusive (due to limited number of experiments and lack of quantitative measurement) and further investigation is needed.
4. Conclusions Wafer grinding possesses several advantages over lapping when used to flatten wire-sawn silicon wafers. Conventional wafer grinding, however, cannot effectively remove wire-sawing induced waviness. Among several techniques proposed to solve this problem, softpad grinding has been shown to be the most promising [27]. Reports of FEA on soft-pad grinding of wiresawn silicon wafers have recently started appearing in the literature, but no systematic experimental investigation has ever been reported. This paper, for the first time, has presented the results of an experimental study on soft-pad grinding of wire-sawn silicon wafers. The major conclusions are: 1. Soft-pad grinding is effective in removing wire-sawing induced waviness.
[1] P. Van Zant, Microchip Fabrication, McGraw-Hill, New York, NY, 2000, p. 37. [2] M. Tricard, S. Kassir, P. Herron, Z.J. Pei, New abrasive trends in manufacturing of silicon wafers, Proceedings of Silicon Machining Symposium, American Society For Precision Engineering, St. Louis, MO, April, 1998. [3] Silicon wafer shipments, revenue fell sharply in 2001, East Bay Business Times, February 12, 2002, Available from. [5] M.S. Bawa, E.F. Petro, H.M. Grimes, Fracture strength of large diameter silicon wafers, Semiconductor International 18 (11) (1995) 115–118. [6] T. Fukami, H. Masumura, K. Suzuki, H. Kudo, Method of manufacturing semiconductor mirror wafers, European Patent Application EP0782179A2, 1997. [7] Z.J. Pei, A study on surface grinding of 300 mm silicon wafers, International Journal of Machine Tools and Manufacture 42 (3) (2002) 385–393. [8] Z.J. Pei, A. Strasbaugh, Fine grinding of silicon wafers, International Journal of Machine Tools and Manufacture 41 (5) (2001) 659–672. [9] Z.J. Pei, A. Strasbaugh, Fine grinding of silicon wafers: designed experiments, International Journal of Machine Tools and Manufacture 42 (3) (2002) 395–404. [10] H.K. Tonshoff, W.V. Schmieden, I. Inasaki, W. Konig, G. Spur, Abrasive machining of silicon, Annals of CIRP 39 (2) (1990) 621–630. [11] R. Vandamme, Y. Xin, Z.J. Pei, Method of processing semiconductor wafers, US Patent 6,114,245, 2000. [12] P.G. Werner, I.M. Kenter, Comparative study of advanced slicing techniques for silicon, in: S. Chandrasekar, R. Komanduri, W. Daniels, W. Rapp (Eds.), Proceedings of Intersociety Symposium on Machining of Advanced Ceramic Materials and ComponentsThe American Society of Mechanical Engineers, New York, NY, 1988. [13] A. Buttner, I.D. sawing—diameters increase, Industrial Diamond Review (2) (1985) 77–79. [14] T. Kato, H. Masumura, S. Okuni, H. Kudo, Method of manufacturing semiconductor wafers, European patent application EP0798405A2, 1997. [15] N. Yasunaga, M. Takashina, T. Itoh, Development of sequential grinding-polishing process applicable to large-size Si wafer finishing, Proceedings of the International Symposium on
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[28] Z.J. Pei, G. Fisher, Surface grinding in silicon wafer manufacturing, Transactions of North American Manufacturing Research Institute SME 29 (2001) 279–286. [29] G.J. Pietsch, M. Kerstan, Simultaneous double-disk grindingmachining process for flat, low-damage and material-saving silicon wafer substrate manufacturing, Proceedings of the Second International Conference of the European Society for Precision Engineering and Nanotoechnology, Turin, Italy, May 27–31 Euspen, Bedford, UK, 2001, pp. 644–648. [30] X.J. Xin, Z.J. Pei, W.J. Liu, Finite element analysis on softpad grinding of wire-sawn silicon wafers, CD-ROM Proceedings of the 2002 International Mechanical Engineering Congress and Exposition, New Orleans, LA, November 17–22, vol. 2, 2002. [31] J. Wu, X.K. Sun, Z.J. Pei, X.J. Xin, Soft-pad grinding of wiresawn silicon wafers: finite element analysis using 25 factorial design, CD-ROM Proceedings of the 12th Annual Industrial Engineering Research Conference (IERC-2003), Portland, Oregon, May 18–20, 2003. [32] S. Hahn, K. Kugimiya, M. Yamashita, P.R. Blaustein, K. Takahashi, Characterization of mirror-like wafer surfaces using the magic mirror method, Journal of Crystal Growth 103 (1–4) (1990) 423–432. [33] S. Hahn, K. Kugimiya, K. Vojtechovsky, M. Sifalda, M. Blaustein, P.R. Blaustein, K. Takahashi, Characterization of mirrorpolished Si wafers and advanced Si substrate structures using the magic mirror method, Semiconductor Science and Technology 7 (1A) (1992) 80–85. [34] C.C. Shiue, K.H. Lie, P.R. Blaustein, Characterization of deformations and texture defects on polished wafers of III–V compound crystals by the magic mirror method, Semiconductor Science and Technology 7 (1A) (1992) 95–97. [35] Z.J. Pei, X.J. Xin, W.J. Liu, Finite element analysis for grinding of wire-sawn silicon wafers: a designed experiment, International Journal of Machine Tools and Manufacture 43 (1) (2003) 7–16. [36] X.K. Sun, Z.J. Pei, X.J. Xin, M. Fouts, Waviness removal in grinding of wire-sawn silicon wafers: 3D finite element analysis with designed experiments, International Journal of Machine Tools and Manufacture, 44(1) (2004) 11–19.
An experimental investigation into soft-pad grinding of wire-sawn silicon wafers Z.J. Pei a,, S. Kassir b, Milind Bhagavat c, Graham R. Fisher c a
Department of Industrial and Manufacturing Systems Engineering, Kansas State University, 237 Durland Hall, Manhattan, KS 66506-5101, USA b Strasbaugh, Inc., 825 Buckley Road, San Luis Obispo, CA 93401, USA c MEMC Electronic Materials, Inc., 501 Pearl Drive, St. Peters, MO 63376, USA Received 11 August 2003; received in revised form 8 September 2003; accepted 16 September 2003
Abstract Silicon is the primary semiconductor material used to fabricate microchips. A series of processes are required to manufacture high-quality silicon wafers. Surface grinding is one of the processes used to flatten wire-sawn wafers. A major issue in grinding of wire-sawn wafers is reduction and elimination of wire-sawing induced waviness. Results of finite element analysis have shown that soft-pad grinding is very effective in reducing the waviness. This paper presents an experimental investigation into soft-pad grinding of wire-sawn silicon wafers. Wire-sawn wafers from a same silicon ingot were used for the study to ensure that these wafers have similar waviness. These wafers were ground using two different soft pads. As a comparison, some wafers were also ground on a rigid chuck. Effectiveness of soft-pad grinding in removing waviness has been clearly demonstrated. # 2003 Elsevier Ltd. All rights reserved. Keywords: Grinding; Lapping; Machining; Material removal; Semiconductor material; Silicon wafer; Slicing
1. Introduction 1.1. Manufacturing processes for silicon wafers Semiconductors are widely used in every type of microelectronic applications, including computer systems, telecommunications equipment, automobiles, consumer electronics, industrial automation and control systems, and analytical and defense systems. The majority of semiconductors are built on silicon wafers [1]. About 150 million silicon wafers of different sizes are manufactured each year worldwide [2]. In 2000, worldwide revenue generated by silicon wafers was US$ 7.5 billion [3]. Semiconductor devices built on these wafers generated US$ 200 billion in revenues [4]. To manufacture high-quality silicon wafers, a sequence of processes is required. As shown in Fig. 1, a typical manufacturing process flow includes the following major processes [5–11]. Note that some processes Corresponding author. Tel.: +1-785-532-3436; fax: +1-785-5323738. E-mail address: [email protected] (Z.J. Pei).
0890-6955/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2003.09.006
are omitted in Fig. 1 for simplicity, for example, edge grinding, edge polishing, and laser marking. 1. Crystal growth—to produce crystal ingots; 2. Slicing (ID sawing or wire sawing)—to slice the ingots into wafers of a thin disk shape; 3. Flattening (lapping or grinding)—to flatten the wafer surface; 4. Etching—to chemically remove processing-induced damage without introducing further mechanical damage; 5. Polishing—to obtain a mirror surface; and 6. Cleaning—to remove the polishing agent or dust particles. 1.2. Pros and cons of wire sawing Until recently, internal-diameter (ID) sawing had been the dominant slicing method for the past three decades [12,13]. Wire sawing, however, is now fully established as the preferred method of slicing large diameter ingots. Although the time per cut in wire sawing is longer than that in ID sawing, the overall throughput
300
Z.J. Pei et al. / International Journal of Machine Tools & Manufacture 44 (2004) 299–306
the ingot, providing a flat reference plane for subsequent processes [17]. 1.3. Pros and cons of grinding Conventionally, wafers after slicing will go through lapping operations for flattening. In a double side lapping operation, batch of wafers (for example, 20 wafers) are manually loaded into a lapping machine. Aluminum-oxide slurry is injected between two metal plates that rotate in opposite directions [18,19]. Despite its widespread use, lapping has the following drawbacks: labor intensive, expensive consumable (slurry), and time consuming. Grinding can be used for flattening sliced wafers to replace or partially replace lapping [11,14,20–23]. The advantages of grinding over lapping include:
Fig. 1. Typical process flow for manufacturing silicon wafers.
of wire sawing is superior because wire sawing can slice hundreds of wafers per cut, while ID sawing cuts one wafer at a time. Furthermore, owing to thinner kerf losses, wire-sawing yields more wafers per unit length of crystal ingot than ID sawing. An undesirable phenomenon associated with wire sawing is the waviness induced by the sawing process. The waviness is also called long cycle swelling or unevenness [14], or wavy stripes [15]. Typically, the wavelength ranges from 0.5 to 30 mm [14]. The generating mechanism of this waviness is not fully understood yet. This has been the main reason that it is very difficult to eliminate waviness at wiresawing operation. If subsequent processes do not remove the waviness, it will adversely affect the flatness, especially the site flatness, of the wafers. A frequently used parameter to measure site flatness is SBIR (site flatness, back reference surface, ideal reference plane, range) [16]. It is the sum of the maximum positive and negative deviations of the surface in a certain area of the wafer from a theoretical reference plane that is approximately parallel to the back surface of the wafer and intersects the front surface at the center of the area. Typical size of the area is 20 20 mm2 for ordinary wafers and 30 35 mm2 for advanced applications. Waviness has not been a problem for ID sawing. With modern ID sawing technology, the backside of the wafer is ground before the wafer is sliced off from
. Grinding is fully automatic. In lapping, wafer loading and unloading is done manually, not only increasing labor costs, but also causing frequent wafer breakage. . Grinding uses fixed-abrasive wheels instead of loose abrasive slurry, hence the cost of consumables per wafer is lower. Furthermore, fixed-abrasive grinding wheels are more benign to the environment than lapping slurry. . Grinding cuts one wafer at a time instead of batch processing. In batch operations, the work in process (WIP) inventory is higher; and once problems occur in production, more defects will be produced before the problems are fixed. Furthermore, a thicknesssorting step is often needed before lapping to ensure that only wafers with small wafer-to-wafer thickness variation are put on the same lapping machine. Often times, as there are not enough production wafers to fill in all the lapping carriers on one machine, extra dummy wafers have to be used, hence adding more cost. . Grinding has higher throughput. For example, a typical lapping operation will take about 40 min to reduce the wafer thickness (200 or 300 mm in diameter) by about 80 lm, while it takes about 30–60 s for grinding to reduce the wafer thickness (200 or 300 mm in diameter) by about 45–75 lm [11]. Grinding has its drawbacks too. It has been reported that lapping can effectively remove waviness but grinding cannot [14,15,24]. Differences in waviness removal by grinding and lapping have been investigated using finite element analysis (FEA) [24]. The results have revealed that, under the same applied force, the relative peak displacement (the displacement of waviness peaks relative to the waviness valleys) at the deformed state in lapping is much smaller, only 1/55 to 1/36 of that in grinding. Smaller relative peak displacement is desir-
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Fig. 3.
Fig. 2. Deformation of a wafer during rigid-chuck grinding and return to its original shape.
able for reducing or eliminating waviness. If a wavy wafer deforms elastically during machining (grinding or lapping), after the operation it will spring back to its original shape thus preserving the waviness. This is illustrated in Fig. 2. 1.4. Approaches to waviness reduction in wafer grinding Wafer grinding is potentially a more cost-effective process than lapping for flattening wire-sawn silicon wafers. However, its success in replacing lapping depends critically on whether the wire-sawing induced waviness can be removed. Several techniques to remove or reduce waviness have been proposed, including wafer grinding followed by lapping [11], reduced chuck vacuum [25], use of ‘‘soft-pad’’ [26], and wax mounting [14]. FEA modeling has shown that soft-pad grinding is the most promising approach because it is effective in reducing the waviness and easily implemented [27]. 1.5. Literature review on soft-pad grinding Soft-pad grinding first appeared in public domain in 1999 through an US patent by Kassir and Walsh [26]. When grinding the first side of a wire-sawn wafer, a perforated resilient pad is inserted in between the wafer and the ceramic chuck (a rigid chuck). The soft pad accommodates and supports the wavy surface of the wafer and holds the wafer in an undeformed condition. As a result, the waviness of the top surface is removed effectively by grinding. This ground surface will be the flat reference plane for grinding the other side of the wafer on a conventional ceramic chuck. The process is illustrated in Fig. 3.
301
Illustration of soft-pad grinding.
In 2001, both Pei and Fisher [28] and Pietsch and Kerstan [29] briefly reported experimental evidence of soft-pad grinding. However, systematic experimental studies on soft-pad grinding of wire-sawn wafers are not available in the literature. A two-dimensional (2D) FEA study [30] showed that soft-pad grinding is effective in removing waviness. Use of a soft pad in between the wafer and the rigid chuck significantly reduces the elastic deformation of the wafer in comparison with grinding on a rigid chuck. This 2D FEA modeling was used to predict the effects of several parameters (such as Young’s modulus and Poisson’s ratio of the soft pad) on the effectiveness of waviness removal. In a follow-up study [31], a 25 (fivefactor two-level) full factorial design was employed to reveal the main effects as well as the interaction effects of five factors (waviness wavelength of silicon wafers; Young’s modulus, Poisson’s ratio, and thickness of soft pads; and grinding force) on effectiveness of waviness reduction. Major conclusions from the FEA investigations include: 1. Waviness wavelength: the longer the wavelength, the more difficult to remove the waviness. 2. Wafer thickness: the thinner the wafer, the more difficult to remove the waviness. 3. Grinding force: the larger the grinding force, the more difficult to remove the waviness. 4. Soft pad properties: it becomes more difficult to remove the waviness as the pad’s Young’s modulus increases, or as the pad’s Poisson’s ratio increases, or as the pad thickness decreases. 1.6. Outline of this paper This paper, for the first time, presents an experimental investigation into soft-pad grinding of wiresawn silicon wafers. Wire-sawn wafers from a same
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silicon ingot were used for the investigation to ensure that these wafers have similar waviness. These wafers were ground using two different soft pads. As a comparison, some wafers were also ground on a rigid chuck. For each of these conditions (with pad A, with pad B, and without any soft pad), two different process sequences were tested. The experimental results have provided verifications for some earlier predictions through the FEA. There are four sections in this paper. Following this Introduction section, Section 2 describes experimental conditions and procedures. In Section 3, experimental results will be presented and discussed. Finally, conclusions are drawn in Section 4.
2. Experimental conditions and procedures Grinding experiments were conducted on a Strasbaugh grinder of Model 7AF. Fig. 4 illustrates the wafer grinding process. Grinding wheels are diamond cup wheels. The workpiece (wafer) is held on a porous ceramic chuck (a rigid chuck) by means of vacuum for conventional grinding. For soft-pad grinding, a perforated soft pad is inserted in between the wafer and the ceramic chuck. The rotation axis for the grinding wheel is offset by a distance of the wheel radius relative to the rotation axis for the wafer. During grinding, the grinding wheel and the wafer rotate about their own
rotation axes simultaneously, and the wheel is fed towards the wafer along its axis. Grinding wheels used were resin-bonded diamond wheels, with diameter of 280 mm. The grit size for the coarse wheel was mesh #320. The grit size for the fine wheel is mesh #2000. Single crystal silicon wafers of 200 mm in diameter with ð1 0 0Þ planes as major surfaces were used for this investigation. All the test wafers were sliced (by wire sawing) at same time from a same crystal ingot, hence had similar waviness (wavelength and height). Grinding parameters and their values are listed in Table 1. Note that there were three feedrate values, used for three sequent steps, respectively. During grinding, deionized (purified) water was used to cool the grinding wheel and the wafer surface. The coolant was supplied to the inner side of the cup wheel, at a flow rate of 3 gal/min. Two types of soft pads (A and B) were tested. The diameter of these pads was 200 mm. Small holes were perforated in both pads to allow vacuum to go through so that the wafer could be held on the ceramic chuck (with the soft pad in between). Both pads have the same nominal mechanical properties and thickness. Detailed information on the pad properties is proprietary. The major difference between the two pads was the arrangement of the perforated holes. Tables 2 and 3 show two grinding sequences: ‘‘flip once’’ and ‘‘flip twice’’, respectively. One group of wafers were ground following the ‘‘flip once’’ sequence, and another group of wafers were ground following the ‘‘flip twice’’ sequence. Under each grinding condition, two or more wafers were ground. However, only one wafer from each grinding condition was chosen for single-side polishing. Same amount of material (15 lm thick) was polished off from the same side of every wafer, using an identical polishing recipe. These polished wafers were inspected under a magic mirror. Pictures of the polished surfaces were taken by the magic mirror to compare the residual waviness. Information on the magic mirror technology can be found in [32–34] and also at website
3. Results and discussion The experimental results are summarized in Table 4. They will be discussed together with some analytical results published before. 3.1. Grinding with a soft pad versus without a soft pad
Fig. 4. Illustration of wafer grinding.
Two wafers (#5 and #6 in Table 4) ground without a soft pad clearly exhibit visible waviness. On the
Z.J. Pei et al. / International Journal of Machine Tools & Manufacture 44 (2004) 299–306 Table 1 Grinding parameters and their values Parameter
Unit
Coarse grinding
Fine grinding
Removal Wheel speed Chuck speed Feedrate for step 1 Feedrate for step 2 Feedrate for step 3
lm rev s1 (rpm) rev s1 (rpm) lm s1
20 32.05 (1923) 1.67 (100) 4
17 72.50 (4350) 9.83 (590) 1
lm s1
0.5
0.5
lm s1
0.3
0.3
Table 2 ‘‘Flip once’’ sequence Operation
Description
1
Coarse and fine grinding the first side of the wafer on a soft pad Flip the wafer Coarse and fine grinding the second side of the wafer on a rigid chuck
2 3
Table 3 ‘‘Flip twice’’ sequence Operation
Description
1
Coarse grinding the first side of the wafer on a soft pad Flip the wafer Coarse and fine grinding the second side of the wafer on a rigid chuck Flip the wafer Fine grinding the first side of the wafer on a rigid chuck
2 3 4 5
contrary, all four wafers (#1 to #4 in Table 4) ground with soft pads do not show any waviness. The effectiveness of soft pads in removing wire-sawing induced waviness is clearly demonstrated. This experimental observation has substantiated the simulation results by an earlier FEA study [30]. As shown in Fig. 5, use of a soft pad can significantly reduce the relative peak displacement (the displacement of waviness peaks relative to the waviness valleys, measuring the reduction of the vertical peak-to-valley distance), and hence can effectively reduce waviness. Simulations by the FEA have shown that, under a specific grinding condition, when grinding without a soft pad, the relative peak displacement reached 20 lm (i.e., the waviness peaks touched the ceramic chuck), hence the waviness cannot be removed. Under the same grinding condition, when grinding with a soft pad of certain properties (Young’s modulus ¼ 1 MPa and Poisson’s ratio ¼ 0:1, and pad thickness ¼ 1 mm), the
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relative peak displacement was only 0.1 lm (almost the same amplitude as the original waviness), hence the vast majority of the waviness will be removed (the survived waviness will have an amplitude of about 0.1 lm). 3.2. ‘‘Flip once’’ sequence versus ‘‘flip twice’’ sequence Effects of sequence (‘‘flip once’’ versus ‘‘flip twice’’) are more obvious for grinding without soft pads than with soft pads. It can be seen that more waviness strips are visible on wafer #5 (ground by ‘‘flip once’’) than on wafer #6 (ground by ‘‘flip twice’’). In other words, when the ‘‘flip once’’ sequence was used (wafer #5), more waviness strips survived the grinding operation. This observation substantiates the FEA predictions published earlier in this journal [35,36]. The FEA simulation of wire-sawn wafer grinding on a rigid chuck [35] and on a soft pad [36] has shown that waviness with longer wavelength is more difficult to remove. It has also been pointed out that waviness is more difficult to remove with fine grinding than with coarse grinding, because the grinding force is much lower in coarse grinding of silicon wafers than in fine grinding. The ‘‘flip twice’’ sequence only employed coarse grinding to the first side of the wafer on the soft pad, while in the ‘‘flip once’’ sequence, both coarse grinding and fine grinding were applied to the first side. Therefore, more waviness strips survived the grinding with the ‘‘flip once’’ sequence. This experimental observation supports a practical implication predicted from an FEA study of wire-sawn wafer grinding on a rigid chuck [35]. In that study, it was shown that as grinding force increased, relative peak displacement increased. In other words, it was easier to remove the wire-sawing induced waviness when the grinding force was lower. Normally, the grinding force in coarse grinding was much lower than that in fine grinding. Therefore, it was predicted that the ‘‘flip twice’’ sequence (it only involves coarse grinding to grind the first side of the wafer) should be more effective than the ‘‘flip once’’ sequence (it involves both coarse and fine grinding to grind the first side of the wafer) in removing the wire-sawing induced waviness. Effects of sequence (‘‘flip once’’ versus ‘‘flip twice’’) on waviness removal are hardly visible for grinding with soft pads. All four wafers (#1 to #4 in Table 4) ground with soft pads do not show any waviness. However, these wafers exhibit other non-uniform patterns (‘‘clouds’’ patterns). These ‘‘clouds’’ patterns have been briefly mentioned by Pei and Fisher [28], and Pietsch and Kerstan [29]. They are the reflection of surface undulations. These ‘‘clouds’’ patterns are probably caused by the non-uniformity of the soft pads. It looks like that the ‘‘clouds’’ patterns on the wafers (#2 and #4) ground by the ‘‘flip twice’’ sequence are less severe
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Table 4 Experimental results Flip once
Flip twice
Soft pad A
Wafer #1
Wafer #2
Soft pad B
Wafer #3
Wafer #4
Wafer #5
Wafer #6
No soft pad
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2. From the viewpoint of removing waviness, ‘‘flip twice’’ sequence is better than ‘‘flip once’’ sequence. 3. Both soft pads under this study are effective in removing waviness. ‘‘Clouds’’ patterns appeared on wafers ground on both soft pads, and further investigations into the ‘‘clouds’’ patterns are needed.
Acknowledgements This work was supported in part by the National Science Foundation under grant DMI-0218237 and by the Advanced Manufacturing Institute of Kansas State University. Fig. 5. FEA simulated comparison between soft-pad grinding and rigid-chuck grinding.
References than those on wafers (#1 and #3) ground by the ‘‘flip once’’ sequence. However, this observation is not conclusive (due to limited number of experiments and lack of quantitative measurement) and further investigation is needed. 3.3. Effects of different pads From Table 4, it can be seen that both pads are effective in removing waviness. Furthermore, the ‘‘clouds’’ patterns on the wafers (#1 and #2) ground by pad A are less severe (that is, the ‘‘clouds’’ areas are less different from other areas in term of darkness/ brightness) than those on wafers (#3 and #4) ground by pad B. Again, this observation is not conclusive (due to limited number of experiments and lack of quantitative measurement) and further investigation is needed.
4. Conclusions Wafer grinding possesses several advantages over lapping when used to flatten wire-sawn silicon wafers. Conventional wafer grinding, however, cannot effectively remove wire-sawing induced waviness. Among several techniques proposed to solve this problem, softpad grinding has been shown to be the most promising [27]. Reports of FEA on soft-pad grinding of wiresawn silicon wafers have recently started appearing in the literature, but no systematic experimental investigation has ever been reported. This paper, for the first time, has presented the results of an experimental study on soft-pad grinding of wire-sawn silicon wafers. The major conclusions are: 1. Soft-pad grinding is effective in removing wire-sawing induced waviness.
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