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High efficiency fine boring of monocrystalline silicon ingot by electrical discharge machining
Precision Engineering 23 (1999) 126 –133
Technical note
High efficiency fine boring of monocrystalline silicon ingot by electrical discharge machining Yoshiyuki Unoa*, Akira Okadaa, Yasuhiro Okamotoa, Kazuo Yamazakib, Subhash H. Risbudb, Yoshiaki Yamadac a
Okayama University, 3-1-1 Tsushimanaka, Okayama 700-8530, Japan b University of California at Davis, Davis, CA 95616-5294, USA c KASEN USA Corp., Charlotte, NC 28273, USA
Received 22 June 1998; received in revised form 20 October 1998; accepted 17 November 1998
Abstract This article deals with high efficiency and high accuracy fine boring in a monocrystalline silicon ingot by electrical discharge machining (EDM). In manufacturing process of integrated circuits, a plasma-etching process is used for removing oxidation films. This process has recently been examined for use of monocrystalline silicon as the electrode to minimize the contamination. However, it is difficult to machine silicon accurately by the conventional diamond drilling method, because the material removal is due to brittle fracture. The machining force in the EDM process is very small compared with that in conventional machining, therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated. The removal rate of monocrystalline silicon by EDM is much higher than that of steel, while the electrode wear is extremely small. The improvement method leads to a better hole without chipping at the exit of hole or sticking of the insulator on the wall of hole. Furthermore, it is proved that even a high aspect ratio of about 200 boring is possible. © 1999 Elsevier Science Inc. All rights reserved. Keywords: Electrical discharge machining; Monocrystalline silicon; Fine boring; High aspect ratio; Contamination
1. Introduction Monocrystalline silicon is one of the most important materials in the semiconductor industry. In the manufacturing process of integrated circuits, the plasma etching process is used for removing oxidation films such as SiO2 [1]. In the process, a graphite plate with many fine holes has been used as an electrode (see Fig. 1) [2,3]. A monocrystalline silicon as the electrode has recently be used to minimize contamination. A diamond grinding quill has generally been used for boring silicon. However, it is difficult to machine accurately with conventional methods, because the material removal is due to brittle fracture. In the diamond drilling method, the removal rate decreases and the loading of tool becomes severe with increasing of the machining depth. Furthermore, the chipping of workpiece occurs at the exit of the hole. * Corresponding author. Tel.: 181-86-251-8037; fax: 181-86-2559669 E-mail address: [email protected] (Y. Uno) 0141-6359/99/$ – see front matter © 1999 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 9 8 ) 0 0 0 2 9 - 4
Fig. 1. General scheme of plasma etching process.
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Fig. 2. Schematic diagram of experimental apparatus.
In this study, electrical discharge machining (EDM) is proposed for boring fine holes in monocrystalline silicon [4,5]. The machining force in the EDM process is extremely small compared with that in conventional machining [6], and slicon wafers used as substrate for epitaxial growth film have low resistivity [7], making it possible to machine silicon by EDm. Therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated.
Experimental procedures Fig. 2 shows the experimental apparatus. Experiments are performed using an NC fine boring EDM machine with
transistor switching circuit. It is also equipped with a condenser circuit. A copper pipe of 1 mm in diameter (inner diameter: 0.3 mm) is used as an electrode, which rotates at 90 rpm. The nonflammable type machining fluid is spouted out from the tip of the electrode at a pressure of 7.8 3 106 Pa. A P-type monocrystalline silicon ingot used as an electrode in plasma etching process has low resistivity in the order of 0.01 Vzcm, which makes it possible to machine silicon by EDM. Boring is done perpendicularly to a crystal face (100). Mild steel SS400 in JIS specification is also employed as a workpiece for comparison. As shown in Fig. 3, a silicon ingot of 5 mm in thickness is mounted with interposing copper plates in order to improve conduction of electric current. A laser displacement sensor is set for in-process measurement of the electrode
Fig. 3. Mounting method of monocrystalline silicon ingot.
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Table 1 Polarity Discharge current Pulse duration Duty factor Capacitance Machining fluid
Electrode (1) Ip 5 2, 6, 15A t p 5 4, 8, 12, 20, 28 ms D.F. 5 50% C 5 380 pF Nonflammable type fluid
movement. The signal of displacement is amplified and recorded by digital recorder. The machining conditions are shown in Table 1. The polarity of electrode is kept positive through the experiment because the removal rate is higher than the reverse case. Capacitance is set at 380 pF, since machining is stabler and a better hole can be obtained compared with any other capacitances. Fig. 5. Relationships between electrode wear in length and pulse duration.
Machining properties Fig. 4 shows the relationships between the removal rate and the pulse duration in EDM of monocrystalline silicon and mild steel SS400. Here, the removal rate is expressed as the ratio of workpiece thickness to the total machining time. As shown in the Fig. 4, the removal rate of silicon is about four to eight times larger than that of steel when compared under the same machining conditions. In EDM, it is generally proved that the removal rate is controlled by the product of melting point um and heat conductivity l, and low value of umzl leads to high removal rate [8]. The value of umzl for silicon is 14.1 3 105 W/m, while that for steel is 1.04 3 105 W/m. From this point of view, the removal rate of steel is expected to be much larger than that of silicon. However, the experimental result shows the opposite tendency. It was recently reported by Saeki et al. [9] that the Joule’s heat generation played an important role in material removal in EDM of high resistance material. Also in this case, it is considered that the Joule’s heat generation in addition to
heat conduction from arc column makes a great contribution to increasing the removal rate of monocrystalline silicon. Therefore, EDM is a very efficient method to machine monocrystalline silicon. Fig. 5 shows the relationships between the electrode wear in length and the pulse duration. The electrode wear is calculated as the ratio of electrode wear length to workpiece thickness. As can be seen from the figure, the electrode wear is about 30% for any pulse durations in the case of steel. On the other hand, the electrode wear in the case of silicon is 1/15 to 1/7 compared with that of steel under the same conditions. Also from this result, it is estimated that the removal volume of monocrystalline silicon by a single pulse discharge is much larger than that of steel, since Joule’s heat generation plays a great role in material removal. Consequently, the electrode wear is very small in the case of monocrystalline silicon. Fig. 6 shows the relationships between the surface roughness of a machined hole and the pulse duration. In cases of
Fig. 4. Relationships between removal rate and pulse duration.
Fig. 6. Relationships between surface roughness and pulse duration.
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Fig. 7. Cross sections of machined hole at exit side.
Fig. 9. SEM micrographs of the cross sections of machined hole.
Ip 5 2A and 6A, the surface roughness is almost constant. On the other hand, it decreases with an increase of pulse duration in the case of Ip 5 15A. Fig. 7 shows the SEM micrographs of the cross sections of machined hole at exit side under large discharge current condition: Ip 5 15A. In the case of t p 5 4 ms, craters generated by a single pulse discharge cannot be observed clearly and the machined surface has larger undulation compared with that in the case of t p 5 28 ms. When pulse duration is short, the diameter of arc column is small and the electric current density is large. Therefore, it is considered that Joule’s heat generation has a great effect on the material removal in this case. Furthermore, brittle fracture due to frequent impact force of electrical discharge seems to have occurred.
during machining time in EDM of monocrystalline silicon. The machining process is divided into three distinct regions with the machining time: normal, stagnation, and quick feed. In the normal region, the machining is stable and the machined depth increases linearly with the machining time. When the electrode advances around the exit of hole, the stagnation region appears. In this region, the electrode retracts suddenly and the machining becomes instable. After the stagnation region, the boring is finished and the electrode advances at the rapid feed speed. Fig. 9 shows the SEM micrographs of the cross sections of machined hole. Good machined surface is generated around the entrance of the hole, while sticking of the insulator is observed on the wall of hole around the exit. Then, EPMA (Electron Probe MicroAnalyzer) analysis of the insulator was carried out. Fig. 10 shows SEM image and element mappings of the inside surface of hole around the entrance and exit. As shown in the figure, silicon, copper, and oxygen exist in both cases. In particular, a lot of oxygen exists on the insulator around the exit. Therefore, it is estimated that the insulator contains silicon oxide and copper oxide. Fig. 11 is the explanation diagrams of these phenomena. As shown in Fig. 11a, the exclusion of debris is smoothly performed through the gap between the workpiece and the electrode until the hole is penetrated. However, once the hole is penetrated as shown in Fig. 11b, the exclusion of debris becomes difficult and they stagnate around the exit because the machining fluid leaks through the hole. The stagnation of debris causes the secondary discharge between the debris and the workpiece, and it makes the insulator stick on the wall of hole around the exit.
Stagnation of machining around exit Fig. 8 shows an example of the displacement of electrode
Improvement method of machining Fig. 8. Relationships between displacement of electrode and machining time.
To maintain the flow of the machining fluid and make the exclusion of debris smooth even around the exit of hole, the
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Fig. 10. SEM images and element mappings of machined surface.
improvement method is introduced, as shown in Fig. 12. In this method, the copper plate of 1 mm in thickness is attached under the silicon ingot firmly. Fig. 13 shows the micrographs of the machined hole generated with this method. A better hole can be obtained
without sticking of the insulator and chipping around the exit. Fig. 14 shows the relationship between the displacement of electrode and the machining time. As can be seen from the figure, there is no stagnation region in this method. It is assumed that the flow of the machining fluid could be
Fig. 11. Exclusion of debris in a) normal region and b) stagnation region.
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Fig. 12. Improved method of machining at the exit.
maintained and the exclusion of debris was done smoothly even around the exit of hole by this method. Then, a better hole can be obtained as shown in the previous figure.
Contamination of inner wall To use this product as the electrode for plasma etching in the silicon process, little contamination of the machined surface should occur. Then, fine boring EDM using copper, silver, and tungsten electrodes of 0.8 mm in diameter was carried out with the improvement method as shown. Fig. 15 shows the SEM micrograph of the machined hole using the
Fig. 13. SEM micrographs of machined hole by improved method.
Fig. 14. Displacement of electrode in improved method.
Fig. 15. SEM micrograph of machined hole using 0.8 tungsten electrode.
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Fig. 16. Result of EDAX analysis.
tungsten electrode. As shown, a fine circular hole with high accuracy can be obtained. Also in the cases of copper and silver electrodes, high accuracy holes can be machined. Next, EDAX (Energy Dispersive X-ray Spectroscopy) analysis was carried out in order to investigate the contamination of inner wall. Fig. 16 shows the result using copper electrode. The only peak seen prominently is that for silicon, while the peak of electrode materials is not detectable. Similar results can be obtained in other cases. It was made clear that this improvement method restrained not only sticking of the insulator but also adhesion or diffusion of electrode material to the machined surface. Therefore, it is considered that this method by EDM is applicable to the fine boring of electrode for plasma etching in the silicon process.
High aspect ratio boring with EDM Thicker electrode of silicon for plasma etching is required in order to reduce the arrangement time. We tried the deep boring of 200 mm silicon which is the diameter length of 8” ingot. Fig. 17 shows the relationship between the displacement of electrode and the machining time. Fig. 18 is the sample of high aspect ratio boring. Under this condition, stable machining is performed at the rate of 1 mm per second and the straightness is maintained. Judging from the experimental result, it seems that a hole deeper than 200 mm is possible to machine efficiently with this method. Generally speaking, it becomes difficult to machine bore a deep hole with a diamond quill, because the loading of the drill or
Fig. 17. Relationship between displacement of electrode and machining time in high aspect ratio boring.
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is attached under the silicon firmly leads to better results. 4. The improvement method restrains not only sticking of the insulator but also adhesion or diffusion of electrode material on machined surface. 5. High aspect ratio boring of monocrystalline silicon is efficiently attained by EDM.
Acknowledgment Fig. 18. Sample of high aspect ratio boring.
the obstruction of debris become severe with the depth of hole. In this method, however, it progresses at the constant speed and it is possible to bore the deep hole whose aspect ratio is about 200. From these results, it is proved that EDM is the suitable method for boring of monocrystalline silicon.
Conclusions Main conclusions obtained in this study are as follows: 1. The removal rate of monocrystalline silicon is much higher than that of steel, while the electrode wear is smaller. 2. The exclusion of debris is difficult around the exit of hole, which leads to the inefficient stagnation region and the sticking of the insulator made from silicon oxide and copper oxide on the wall of hole. 3. The improvement method in which the copper plate
The authors express their thanks to Sin-Etsu Handotai Co. for supplying the workpieces of monocrystalline silicon. References [1] Shono K. Semiconductor Technology. Tokyo: Tokyo-daigaku-shuppankai, 1987, p. 138. [2] Sugano T. Plasma Process Technology for Semiconductor. Tokyo: Sangyo-tosho, 1993, p. 228. [3] Tokuyama T. Dry Etching Technology for Semiconductor. Tokyo: Sangyo-tosho, 1992, p. 83. [4] Dominiek R, et al. Microstructuring of silicon by electrodischarge machining (EDM). Sensors and Actuators 1997;60(1-3):212– 8. [5] Uno Y, et al. Fundamental study on electrical discharge machining of single crystalline silicon. J JSPE 1997;63(10):1459 – 63. [6] Masuzawa T. Micro EDM. J JSPE 199;57(6):963–7. [7] Shimura F. Semiconductor Silicon Crystal Technology. San Diego, CA: Academic Press, 1988, p. 197. [8] Saito N. Principle of EDM and its Application. Tokyo: Gijutsuhyoronsha, 1994, p. 38. [9] Saeki T, et al. Transient workpiece temperature analysis in the EDM processes of high electric resistance materials considering Joule’s heating. J JSPE 1996;62(3):443–7.
Technical note
High efficiency fine boring of monocrystalline silicon ingot by electrical discharge machining Yoshiyuki Unoa*, Akira Okadaa, Yasuhiro Okamotoa, Kazuo Yamazakib, Subhash H. Risbudb, Yoshiaki Yamadac a
Okayama University, 3-1-1 Tsushimanaka, Okayama 700-8530, Japan b University of California at Davis, Davis, CA 95616-5294, USA c KASEN USA Corp., Charlotte, NC 28273, USA
Received 22 June 1998; received in revised form 20 October 1998; accepted 17 November 1998
Abstract This article deals with high efficiency and high accuracy fine boring in a monocrystalline silicon ingot by electrical discharge machining (EDM). In manufacturing process of integrated circuits, a plasma-etching process is used for removing oxidation films. This process has recently been examined for use of monocrystalline silicon as the electrode to minimize the contamination. However, it is difficult to machine silicon accurately by the conventional diamond drilling method, because the material removal is due to brittle fracture. The machining force in the EDM process is very small compared with that in conventional machining, therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated. The removal rate of monocrystalline silicon by EDM is much higher than that of steel, while the electrode wear is extremely small. The improvement method leads to a better hole without chipping at the exit of hole or sticking of the insulator on the wall of hole. Furthermore, it is proved that even a high aspect ratio of about 200 boring is possible. © 1999 Elsevier Science Inc. All rights reserved. Keywords: Electrical discharge machining; Monocrystalline silicon; Fine boring; High aspect ratio; Contamination
1. Introduction Monocrystalline silicon is one of the most important materials in the semiconductor industry. In the manufacturing process of integrated circuits, the plasma etching process is used for removing oxidation films such as SiO2 [1]. In the process, a graphite plate with many fine holes has been used as an electrode (see Fig. 1) [2,3]. A monocrystalline silicon as the electrode has recently be used to minimize contamination. A diamond grinding quill has generally been used for boring silicon. However, it is difficult to machine accurately with conventional methods, because the material removal is due to brittle fracture. In the diamond drilling method, the removal rate decreases and the loading of tool becomes severe with increasing of the machining depth. Furthermore, the chipping of workpiece occurs at the exit of the hole. * Corresponding author. Tel.: 181-86-251-8037; fax: 181-86-2559669 E-mail address: [email protected] (Y. Uno) 0141-6359/99/$ – see front matter © 1999 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 9 8 ) 0 0 0 2 9 - 4
Fig. 1. General scheme of plasma etching process.
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Fig. 2. Schematic diagram of experimental apparatus.
In this study, electrical discharge machining (EDM) is proposed for boring fine holes in monocrystalline silicon [4,5]. The machining force in the EDM process is extremely small compared with that in conventional machining [6], and slicon wafers used as substrate for epitaxial growth film have low resistivity [7], making it possible to machine silicon by EDm. Therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated.
Experimental procedures Fig. 2 shows the experimental apparatus. Experiments are performed using an NC fine boring EDM machine with
transistor switching circuit. It is also equipped with a condenser circuit. A copper pipe of 1 mm in diameter (inner diameter: 0.3 mm) is used as an electrode, which rotates at 90 rpm. The nonflammable type machining fluid is spouted out from the tip of the electrode at a pressure of 7.8 3 106 Pa. A P-type monocrystalline silicon ingot used as an electrode in plasma etching process has low resistivity in the order of 0.01 Vzcm, which makes it possible to machine silicon by EDM. Boring is done perpendicularly to a crystal face (100). Mild steel SS400 in JIS specification is also employed as a workpiece for comparison. As shown in Fig. 3, a silicon ingot of 5 mm in thickness is mounted with interposing copper plates in order to improve conduction of electric current. A laser displacement sensor is set for in-process measurement of the electrode
Fig. 3. Mounting method of monocrystalline silicon ingot.
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Table 1 Polarity Discharge current Pulse duration Duty factor Capacitance Machining fluid
Electrode (1) Ip 5 2, 6, 15A t p 5 4, 8, 12, 20, 28 ms D.F. 5 50% C 5 380 pF Nonflammable type fluid
movement. The signal of displacement is amplified and recorded by digital recorder. The machining conditions are shown in Table 1. The polarity of electrode is kept positive through the experiment because the removal rate is higher than the reverse case. Capacitance is set at 380 pF, since machining is stabler and a better hole can be obtained compared with any other capacitances. Fig. 5. Relationships between electrode wear in length and pulse duration.
Machining properties Fig. 4 shows the relationships between the removal rate and the pulse duration in EDM of monocrystalline silicon and mild steel SS400. Here, the removal rate is expressed as the ratio of workpiece thickness to the total machining time. As shown in the Fig. 4, the removal rate of silicon is about four to eight times larger than that of steel when compared under the same machining conditions. In EDM, it is generally proved that the removal rate is controlled by the product of melting point um and heat conductivity l, and low value of umzl leads to high removal rate [8]. The value of umzl for silicon is 14.1 3 105 W/m, while that for steel is 1.04 3 105 W/m. From this point of view, the removal rate of steel is expected to be much larger than that of silicon. However, the experimental result shows the opposite tendency. It was recently reported by Saeki et al. [9] that the Joule’s heat generation played an important role in material removal in EDM of high resistance material. Also in this case, it is considered that the Joule’s heat generation in addition to
heat conduction from arc column makes a great contribution to increasing the removal rate of monocrystalline silicon. Therefore, EDM is a very efficient method to machine monocrystalline silicon. Fig. 5 shows the relationships between the electrode wear in length and the pulse duration. The electrode wear is calculated as the ratio of electrode wear length to workpiece thickness. As can be seen from the figure, the electrode wear is about 30% for any pulse durations in the case of steel. On the other hand, the electrode wear in the case of silicon is 1/15 to 1/7 compared with that of steel under the same conditions. Also from this result, it is estimated that the removal volume of monocrystalline silicon by a single pulse discharge is much larger than that of steel, since Joule’s heat generation plays a great role in material removal. Consequently, the electrode wear is very small in the case of monocrystalline silicon. Fig. 6 shows the relationships between the surface roughness of a machined hole and the pulse duration. In cases of
Fig. 4. Relationships between removal rate and pulse duration.
Fig. 6. Relationships between surface roughness and pulse duration.
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Fig. 7. Cross sections of machined hole at exit side.
Fig. 9. SEM micrographs of the cross sections of machined hole.
Ip 5 2A and 6A, the surface roughness is almost constant. On the other hand, it decreases with an increase of pulse duration in the case of Ip 5 15A. Fig. 7 shows the SEM micrographs of the cross sections of machined hole at exit side under large discharge current condition: Ip 5 15A. In the case of t p 5 4 ms, craters generated by a single pulse discharge cannot be observed clearly and the machined surface has larger undulation compared with that in the case of t p 5 28 ms. When pulse duration is short, the diameter of arc column is small and the electric current density is large. Therefore, it is considered that Joule’s heat generation has a great effect on the material removal in this case. Furthermore, brittle fracture due to frequent impact force of electrical discharge seems to have occurred.
during machining time in EDM of monocrystalline silicon. The machining process is divided into three distinct regions with the machining time: normal, stagnation, and quick feed. In the normal region, the machining is stable and the machined depth increases linearly with the machining time. When the electrode advances around the exit of hole, the stagnation region appears. In this region, the electrode retracts suddenly and the machining becomes instable. After the stagnation region, the boring is finished and the electrode advances at the rapid feed speed. Fig. 9 shows the SEM micrographs of the cross sections of machined hole. Good machined surface is generated around the entrance of the hole, while sticking of the insulator is observed on the wall of hole around the exit. Then, EPMA (Electron Probe MicroAnalyzer) analysis of the insulator was carried out. Fig. 10 shows SEM image and element mappings of the inside surface of hole around the entrance and exit. As shown in the figure, silicon, copper, and oxygen exist in both cases. In particular, a lot of oxygen exists on the insulator around the exit. Therefore, it is estimated that the insulator contains silicon oxide and copper oxide. Fig. 11 is the explanation diagrams of these phenomena. As shown in Fig. 11a, the exclusion of debris is smoothly performed through the gap between the workpiece and the electrode until the hole is penetrated. However, once the hole is penetrated as shown in Fig. 11b, the exclusion of debris becomes difficult and they stagnate around the exit because the machining fluid leaks through the hole. The stagnation of debris causes the secondary discharge between the debris and the workpiece, and it makes the insulator stick on the wall of hole around the exit.
Stagnation of machining around exit Fig. 8 shows an example of the displacement of electrode
Improvement method of machining Fig. 8. Relationships between displacement of electrode and machining time.
To maintain the flow of the machining fluid and make the exclusion of debris smooth even around the exit of hole, the
130
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Fig. 10. SEM images and element mappings of machined surface.
improvement method is introduced, as shown in Fig. 12. In this method, the copper plate of 1 mm in thickness is attached under the silicon ingot firmly. Fig. 13 shows the micrographs of the machined hole generated with this method. A better hole can be obtained
without sticking of the insulator and chipping around the exit. Fig. 14 shows the relationship between the displacement of electrode and the machining time. As can be seen from the figure, there is no stagnation region in this method. It is assumed that the flow of the machining fluid could be
Fig. 11. Exclusion of debris in a) normal region and b) stagnation region.
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131
Fig. 12. Improved method of machining at the exit.
maintained and the exclusion of debris was done smoothly even around the exit of hole by this method. Then, a better hole can be obtained as shown in the previous figure.
Contamination of inner wall To use this product as the electrode for plasma etching in the silicon process, little contamination of the machined surface should occur. Then, fine boring EDM using copper, silver, and tungsten electrodes of 0.8 mm in diameter was carried out with the improvement method as shown. Fig. 15 shows the SEM micrograph of the machined hole using the
Fig. 13. SEM micrographs of machined hole by improved method.
Fig. 14. Displacement of electrode in improved method.
Fig. 15. SEM micrograph of machined hole using 0.8 tungsten electrode.
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Fig. 16. Result of EDAX analysis.
tungsten electrode. As shown, a fine circular hole with high accuracy can be obtained. Also in the cases of copper and silver electrodes, high accuracy holes can be machined. Next, EDAX (Energy Dispersive X-ray Spectroscopy) analysis was carried out in order to investigate the contamination of inner wall. Fig. 16 shows the result using copper electrode. The only peak seen prominently is that for silicon, while the peak of electrode materials is not detectable. Similar results can be obtained in other cases. It was made clear that this improvement method restrained not only sticking of the insulator but also adhesion or diffusion of electrode material to the machined surface. Therefore, it is considered that this method by EDM is applicable to the fine boring of electrode for plasma etching in the silicon process.
High aspect ratio boring with EDM Thicker electrode of silicon for plasma etching is required in order to reduce the arrangement time. We tried the deep boring of 200 mm silicon which is the diameter length of 8” ingot. Fig. 17 shows the relationship between the displacement of electrode and the machining time. Fig. 18 is the sample of high aspect ratio boring. Under this condition, stable machining is performed at the rate of 1 mm per second and the straightness is maintained. Judging from the experimental result, it seems that a hole deeper than 200 mm is possible to machine efficiently with this method. Generally speaking, it becomes difficult to machine bore a deep hole with a diamond quill, because the loading of the drill or
Fig. 17. Relationship between displacement of electrode and machining time in high aspect ratio boring.
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is attached under the silicon firmly leads to better results. 4. The improvement method restrains not only sticking of the insulator but also adhesion or diffusion of electrode material on machined surface. 5. High aspect ratio boring of monocrystalline silicon is efficiently attained by EDM.
Acknowledgment Fig. 18. Sample of high aspect ratio boring.
the obstruction of debris become severe with the depth of hole. In this method, however, it progresses at the constant speed and it is possible to bore the deep hole whose aspect ratio is about 200. From these results, it is proved that EDM is the suitable method for boring of monocrystalline silicon.
Conclusions Main conclusions obtained in this study are as follows: 1. The removal rate of monocrystalline silicon is much higher than that of steel, while the electrode wear is smaller. 2. The exclusion of debris is difficult around the exit of hole, which leads to the inefficient stagnation region and the sticking of the insulator made from silicon oxide and copper oxide on the wall of hole. 3. The improvement method in which the copper plate
The authors express their thanks to Sin-Etsu Handotai Co. for supplying the workpieces of monocrystalline silicon. References [1] Shono K. Semiconductor Technology. Tokyo: Tokyo-daigaku-shuppankai, 1987, p. 138. [2] Sugano T. Plasma Process Technology for Semiconductor. Tokyo: Sangyo-tosho, 1993, p. 228. [3] Tokuyama T. Dry Etching Technology for Semiconductor. Tokyo: Sangyo-tosho, 1992, p. 83. [4] Dominiek R, et al. Microstructuring of silicon by electrodischarge machining (EDM). Sensors and Actuators 1997;60(1-3):212– 8. [5] Uno Y, et al. Fundamental study on electrical discharge machining of single crystalline silicon. J JSPE 1997;63(10):1459 – 63. [6] Masuzawa T. Micro EDM. J JSPE 199;57(6):963–7. [7] Shimura F. Semiconductor Silicon Crystal Technology. San Diego, CA: Academic Press, 1988, p. 197. [8] Saito N. Principle of EDM and its Application. Tokyo: Gijutsuhyoronsha, 1994, p. 38. [9] Saeki T, et al. Transient workpiece temperature analysis in the EDM processes of high electric resistance materials considering Joule’s heating. J JSPE 1996;62(3):443–7.