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Some experiments on the scratching of silicon:
International Journal of Mechanical Sciences 43 (2001) 335}347
Some experiments on the scratching of silicon: In situ scratching inside an SEM and scratching under high external hydrostatic pressures M. Yoshino , T. Aoki , T. Shirakashi, R. Komanduri * Tokyo Institute of Technology, Tokyo, Japan Tokyo Denki University, Tokyo, Japan School of Mechanical and Aerospace Engineering, Oklahoma State University, 218 Engineering North, Stillwater, OK 74078, USA Received 30 April 1999; received in revised form 17 December 1999; accepted 12 January 2000
Abstract To elucidate the mechanisms of material removal in ultra precision machining of silicon involving deformation and fracture, in situ scratching of silicon with a diamond stylus inside a scanning electron microscope (SEM) using a specially designed tribometer and scratching under zero and high (400 MPa) external hydrostatic pressures using a specially designed high-pressure machining apparatus were conducted. The resulting scratches were examined in an SEM to evaluate the in#uence of depth of cut and hydrostatic pressure on the nature of scratches produced (smooth versus fractured surfaces) and the possible brittle}ductile transition. Experimental results indicate that hydrostatic pressure plays an important role in minimizing fracture and producing smooth surfaces. Reasonable agreement of the experimental results with the meso-plasticity FEM simulation of indentation was obtained. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Silicon is the basic substrate material in the manufacture of the chips in the semiconductor industry. Currently, 200 mm diameter silicon wafers are processed routinely to produce the semiconductor chips. However, global competition is forcing industry to further reduce costs in
* Corresponding author. Tel.: 001-405-744-5900; fax: 001-405-744-7873. E-mail address: [email protected] (R. Komanduri). 0020-7403/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 0 - 7 4 0 3 ( 9 9 ) 0 0 0 1 9 - 9
336
M. Yoshino et al. / International Journal of Mechanical Sciences 43 (2001) 335}347
"nishing silicon. It is anticipated that the next millennium would see the end of the 200 mmdiameter silicon wafer era and the dawn of the larger, 300 mm-diameter silicon wafer era. This is based on the history of the semiconductor industry where utilizing larger diameter wafers were found to result in the reduction of manufacturing costs and increase in the production performance signi"cantly. However, in order to produce 300 mm silicon wafers, it is necessary to accomplish much higher precision and better surface integrity than those with a 200 mm silicon wafer. Such requirements may exceed the performance capabilities of the conventional "nishing technology, namely, lapping/polishing practiced today. Consequently, a new machining technology for the "nishing of larger size silicon wafers is anticipated to replace the conventional "nishing technology. A promising technology towards the production of defect free "nishing of silicon is single point diamond turning using a high precision, extremely rigid machine tool. Some researchers have reported the technical feasibility of single point diamond turning of silicon when the size of cut is below a critical value [1]. They termed it `ductile regimea turning indicating plastic deformation of silicon ahead of the tool forming a shear zone followed by the chip formation akin to the machining of a ductile metal where long continuous chips are generated ahead of the tool. They argue that the material behaves in a ductile manner below the critical depth and in a brittle manner above it. Others, such as Yoshino et al. [2] investigated the possibility of a continuous chip formation by a #ow-mode or a `ductile-modea machining of glass and other hard materials, such as ceramics when the size of the cut was below a critical thickness ((1lm). Puttick et al. [3], however, cautioned on the crucial importance of the resulting surface damage. They reported the mean depth of permanent damage to be in the range of 100}400 nm. Shibata et al. [4] conducted diamond turning tests along the [1 1 0] direction on the (0 0 1) plane of a single-crystal silicon at depths of cuts of 100 and 500 nm. Cross-section TEM revealed conversion of the surface layers (&150 nm thick) to an amorphous structure followed by a plastically deformed layer (2}3 lm thick) with numerous dislocations. They also showed evidence of a typical continuous chip by turning silicon at 100 nm cut depth. At the higher depth of cut, namely, 500 nm, material removal was found to be of brittle nature. The interesting aspect of this work is that they used a tool with a !403 rake. Such high negative rakes are hardly used in conventional machining of ductile materials. However, it appears feasible to machine silicon with single crystal diamond only when large negative rake angle are used (!303 and higher). Such large negative rake angles can induce considerable hydrostatic pressure to plastically deform the material directly underneath the tool. Puttick and Hosseini [5] also investigated the initiation of fracture during indentation and scratching on near (1 1 1) silicon surfaces. Cleavage on +1 1 0, planes rather than on +1 1 1, together with a preference for crack propagation in the surface layers were observed. They proposed that +1 1 0, cleavage is initiated by dislocation interactions. There are other researchers who investigated the fracture and brittle nature of silicon [6,7]. The formation of amorphous silicon was also reported in hardness indentation and scratching after the indenter was unloaded [8}10]. Tanikella et al. [11] investigated microcutting of silicon and observed the presence of amorphous silicon within the cutting grooves as well as in the cutting debris located outside of the grooves. Puttick et al. [12] investigated machining of brittle materials including silicon and various glasses, such as soda-lime glass and fused quartz. They showed macrographs of the machined surfaces of fused quartz (Spectrosil) where many areas contained swarf adhering to the sides of the grooves. They also showed numerous ribbons detaching from one side of the grooves only, leaving a striped structure or tearing a ragged foil alongside the original groove or two "laments of the
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workmaterial peeling o! in opposite directions. Finally, they showed many strips in a state of incipient detachment with loops "xed at both ends. They actually considered that these "laments are generated in the grooves ahead of the tool. These ribbons appear to have been formed not in front of the grooves but on the sides of the grooves as Puttick et al. [12] comment on some of these strips. The delamination and subsequent separation of the "laments led Puttick et al. [12] to conclude that the material on either side of the machined groove is in a state of longitudinal compressive residual stress and subjected to buckling. They also proposed a plastic}elastic peeling e!ect due to the presence of residual stresses. The mechanism of material removal in micromachining of glass, according to Puttick et al. is accomplished by at least two distinct processes, namely, groove formation and subsequent swarf removal, though their relative importance may vary between materials. The fact that the two are not concurrent is particularly noteworthy. It appears that the groove formation is due to indentation and the swarf formation is due to side #ow and subsequent peeling and separation of the "lamentary material during subsequent sliding. To understand the mechanism of material removal in single point ultraprecision machining of silicon, it is necessary to simulate conditions as close to it as possible. Although indentation and scratching process is not exactly equivalent to the machining process, it is commonly used as a "rst step in the modeling of the machining process. In conventional machining, a cutting tool indents into the workmaterial followed by scratching/cutting by the tool. Also, a cylindrical shape of the stylus gives plane strain conditions similar to orthogonal machining. Therefore, the dislocation generation and propagation under plane strain conditions are expected to be similar to the practical scratching/machining process. In the previous investigation [13], meso-plasticity FEM simulation of indentation of single crystal silicon under plane strain conditions was conducted. In this investigation in situ indentation-scratching tests with a diamond stylus inside an SEM using a special tribometer as well as machining under zero and high (400 MPa) external hydrostatic pressure using a specially designed machining apparatus inside a pressure vessel were conducted for a better appreciation of the process.
2. Experiments on scratching of silicon 2.1. In situ scratching inside an SEM using a tribometer Fig. 1(a) is a schematic of a pin-on-disk type tribometer used for the in situ scratching of silicon inside the scanning electron microscope (SEM). The tribometer replaces the normal manually operating rotating stage of the SEM. It is equipped with a small load cell (strain gage-based) to measure the scratch force during the scratch test. Fig. 1(b) shows some details of the load cell. The stylus is mounted on a brass foil on which four strain gages are mounted. From the variation of resistance of these strain gages, both the normal and the tangential forces can be measured independently. On top of the stylus, a small single-crystal diamond pin is located. Fig. 2 is an SEM micrograph of a diamond tip from which the radius of the tip is estimated to be &15 lm. The scratching process conducted inside the SEM was observed in real time and stored using a video recorder. SEM micrographs were also taken at various stages of the scratch process as well as under various scratching conditions.
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Fig. 1. Tribometer used for the in situ scratching of silicon inside the SEM. (b) Detail of the load cell used in the tribometer.
The silicon sample is mounted on the rotating stage of the SEM with a silver conducting cement. The stylus is positioned gently on the workmaterial and the desired normal force is applied by adjusting the sti!ness of a spring attached on the other side of the lever. With the tribometer inside the SEM, the chamber is closed and pumped down to the required vacuum to operate the SEM.
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Fig. 2. SEM micrograph of the diamond pin. The radius of the tip is estimated to be &15 lm.
The position of the stylus is adjusted by the movement of the X}> table from outside the SEM chamber. The rotating stage is driven externally by a motor at a given speed. The motor can be driven in both directions so that the scratching process can be observed both in front of and behind the stylus. The following test conditions were used for in situ scratching inside the SEM : Specimen: single crystal silicon +1 1 1, plane on the surface, Thrust force: 1}3 N, Scratch speed: 265.9 lm/s, Scratch direction: 11 1 02:03,11 1 22:303, Atmosphere: vacuum (10\}10\ Torr), no lubricant. 2.2. Scratching under high hydrostatic pressures: To investigate the e!ect of external hydrostatic pressure on the defect generation in indentation/scratching/machining, a special machining apparatus for conducting scratching under high external hydrostatic pressure (400 MPa) conditions was developed [14]. Fig. 3(a) shows a schematic of the machining apparatus built inside a high hydrostatic pressure vessel. Kerosene was used as the pressurizing #uid. The pressure vessel was made of steel with a design pressure of 500 MPa. The chamber was connected to an external high-pressure pump through the pressure intake. The shaft of the turntable is connected to a variable speed motor. Scratching speeds from 0.8 to 8 mm/s can be obtained with this apparatus. The turntable is supported by an angular contact bearing and leakage around the shaft is prevented using Bridgman seals. Fig. 3(b) shows details of the machining stage. The silicon specimen is attached to the turntable. A diamond stylus
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Fig. 3. (a) Schematic of the experimental setup used in the scratching of silicon under high external hydrostatic pressures: 1. pressure vessel, 2. pressure intake, 3. turntable, 4. angular bearing, 5. Bridgman seal, 6. sleeve, 7. Bridgman seal, 8. lid, and 9. bolts to secure the lid. (b) Details of the scratching apparatus: 10. specimen, 11. tool holder, 12. fulcrum of the tool holder, 13. diamond stylus, 14. spring, 15. spring adjuster.
Fig. 4. SEM micrograph of the diamond stylus. The radius of the tip is estimated to be &17 lm.
is glued to the tool holder with an epoxy bond and is supported by a fulcrum. Fig. 4 is an SEM micrograph of a diamond stylus from which the radius of the tip is estimated to be &17 lm. The scratches formed under di!erent conditions were examined in an SEM to investigate the nature of deformation and fracture.
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3. Results and discussion 3.1. Results of in-situ silicon scratching inside SEM: Figs. 5(a)}(d) are SEM micrographs of the grooves made on the silicon specimens in the 03 scratch direction. Values of the width, w and the depth, d of the grooves are given in the "gure. Depth of the groove, d is calculated using the equation
d"R! R!
w , 2
(1)
Fig. 5. SEM micrographs of the grooves made on the silicon specimens. (a)}(d) Scratch direction is 03. Values of the width, w and depth, d of the grooves are given in the "gures.
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where R is the radius of the pin. It can be seen that smooth grooves can be generated at depths of cut of up to about 0.6 lm after which some cracking can be seen. Also, material plowed to the sides of the groove can be seen. Fig. 6 is the variation of normal force, F with the indentation depth, X showing the e!ect of L normal force on the crack generation by scratching. Solid symbols in the "gure indicate that cracks were generated in the scratching process while open symbols indicate absence of cracks. The critical depth below which no cracks are formed is found to be &0.6 lm. Thus, defect free scratching seems to depend on the normal force (or the depth of the groove). The critical force varies from 1.8 N to 2.0 N while the critical depth of the groove varies from 0.5 to 0.7 lm. As shown in Figs. 5(a)}(c), when the depth of the groove is less than the critical depth, no cracks are formed along the groove; instead many deformed silicon chips are seen. The nature of the chips suggest that silicon may have been deformed plastically. When the depth of the groove is larger than the critical depth [Fig. 5(d)], cracks were generated periodically and broken fragments due to brittle fracture were formed. This phenomena, however, does not seem to depend on the scratching direction based on experimental observations. Fig. 7 shows the relationship between the tangential force and the normal force for 11 1 02 and 11 1 22 directions of scratching. It can be seen that the ratio of tangential to the normal force varies with the rotation angle and the direction of scratching. For the 11 1 02 direction of scratching, the mean friction coe$cient between the pin and the silicon workmaterial is &1.0 (or the friction angle &453) and for the 11 1 22 direction of scratching it is &0.75. 3.2. Results of scratching of silicon under high external hydrostatic pressure: Figs. 8(a) and (b) are SEM micrographs of the grooves made on the silicon specimens at no (or zero) external hydrostatic pressure and at 400 MPa hydrostatic pressure, respectively. They show a smooth surface under high external hydrostatic pressure (400 MPa) and fracture at no (or zero) external hydrostatic pressure. Thus hydrostatic pressure is very e!ective in reducing machining defects, such as microcracks. Fig. 9 shows the relationship between the groove cross-sectional area and the crack ratio. The crack ratio is de"ned as the total length of cracks along unit length of the
Fig. 6. Variation of the normal load with the indentation depth. Solid symbols in the "gure indicate that cracks were generated in the scratching process while open symbols indicate appearance of no cracks. The critical depth below which no cracks are formed is &0.6 lm.
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Fig. 7. Relationship between the tangential force and the normal force for 11 1 02 and 11 1 22 directions of scratching.
Fig. 8. (a) and (b) SEM micrographs of the grooves made on the silicon specimens at zero or no hydrostatic pressure and at 400 MPa hydrostatic pressure.
Fig. 9. Relationship between the groove cross-sectional area and the crack ratio.
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groove. Open circles are for crack ratio under no (or zero) hydrostatic pressure, and solid circles for crack ratio under a hydrostatic pressure of 400 MPa. This "gure shows that the critical value of the groove sectional area, where cracks do not appear, increases by about 10 times under high hydrostatic pressure (400 MPa) than without any external hydrostatic pressure. It can be shown that the origin of these defects is not on the surface but in the interior of the specimen. As shown schematically in Fig. 10(a), if defects in the form of microcracks are present in
Fig. 10. (a) and (b) Schematics indicating the origin of the defects at or near the surface and in the interior, respectively and the in#uence of high hydrostatic pressure on crack propagation or arrest.
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the material, the hydrostatic pressure can prevent crack propagation by closure of these cracks. Thus under the in#uence of high hydrostatic pressure, crack propagation can be arrested and the amount of machining defects can be decreased. On the other hand, if the cracks are present on or near the surface, as illustrated in Fig. 10(b), the pressure medium is forced into the cracks and hydrostatic pressure reaches the tip of the cracks. This hydrostatic pressure balances with the hydrostatic pressure from outside of the specimen, and the e!ect of hydrostatic pressure is cancelled. Hence, the amount of machining defects would not decrease if the defects were attributed to the surface cracks. Therefore, the origin of machining defects must exist in the interior of the workmaterial. Fig. 11 shows variation of defect density during indentation under zero and high (400 MPa) external hydrostatic pressures obtained using the meso-plasticity FEM simulation [13]. It shows that the defect density is reduced to nearly half under the in#uence of high (400 MPa) hydrostatic pressure compared to zero hydrostatic pressure for all values of the indentation depth up to an indentation depth of 2 lm. Also, with increase in the depth of indentation, while the defect density increases rapidly under zero or no hydrostatic pressure, it increases at much lower rate under high (400 MPa) hydrostatic pressure. For example, let us assume the critical defect density, g to be 60 MPa. The critical indentation depth (or critical thickness) for this is &2 lm under zero or no high hydrostatic pressure while it is &5 lm under high (400 MPa) hydrostatic pressure. This shows the e!ect of hydrostatic pressure on critical size of indentation. It, therefore, appears that the generation of defects in the machining of brittle materials, such as silicon are related to the ductile}brittle transition. The machining defects can be attributed not necessarily to the preexisting microcracks in the workmaterial but to the concentration of dislocations generated by plastic deformation under light loads and high hydrostatic pressures immediately underneath the indenter and propagated further into the interior of the workmaterial as indentation progresses. It can be seen that while the trends in the defect structure are somewhat similar for the analytical and the experimental values, there is di!erence in the magnitude. For example, in the experimental value, the critical depth was &0.6 lm while the analytical results indicate a much higher value,
Fig. 11. E!ect of external hydrostatic pressure (400 MPa) on the maximum value of defect density [13].
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namely, &2 lm (Fig. 11). This di!erence can partially be attributed to the fact that only loading (and not the unloading) was considered in this investigation. The unloading could a!ect the critical depth. Also, there are some basic di!erences in the nature of deformation, #ow, and fracture in the indentation and scratching processes.
4. Conclusions 1. Scratching tests were conducted on a single-crystal silicon workmaterial using a pin-on-disctype tribometer inside an SEM and a special scratching apparatus in a high external hydrostatic pressure vessel to study the e!ect of scratch depth and hydrostatic pressures on the nature of scratch generation in silicon. 2. Scratching at various loads indicate brittle fracture above a certain load (&1.8}2 N) and practically no cracking and generation of smooth surfaces below that load. Based on the relationship between the normal load and the indentation depth, the critical depth above which cracks are formed is found to be &0.6 lm. When the depth of the groove is less than the critical depth, no cracks are found along the groove; instead many deformed silicon chips were found suggesting that silicon may have been deformed plastically. When the depth of a groove is larger than the critical depth, cracks are generated periodically and broken fragments were found to come out as a result of brittle fracture. Also, plowing on the sides of the groove can be seen. 3. Scratching tests were conducted on a single-crystal silicon workmaterial using a special machining apparatus built inside a high hydrostatic pressure vessel to study the e!ect of external hydrostatic pressure. Results indicate cracking under zero or no external hydrostatic pressure and practically no cracking and generation of smooth surfaces under high hydrostatic pressure (400 MPa). The critical value of the groove sectional area where cracks do not appear was found to increase by &10 times under high hydrostatic pressure (400 MPa) than without any external hydrostatic pressure. 4. While grooves with smooth surfaces and absence of any fracture can be generated in silicon, the formation mechanism may have no resemblance to the conventional machining with a single point tool, namely, plastic deformation ahead of the tool and formation of long continuous chips. Instead plastic deformation underneath the tool due to high hydrostatic pressure and conversion of silicon from semiconductor to metallic state may be taking place. This is followed by delamination and subsequent separation of the "laments on either side of the machined grooves which are in a state of longitudinal compressive residual stress and subjected to buckling, as proposed by Puttick et al. [12].
Acknowledgements This work was supported by a grant from the US}Japan co-operative program on High E$ciency, Damage-free Finishing of Advanced Ceramics and Glasses (INT-9603002). The authors thank the US National Science Foundation and the Ministry of Education of Japan for support of this work. In particular, one of the authors (R.K.) extends his sincere thanks to Dr. L. Webber of the International Programs of NSF and Drs. L. Matin-Vega, B.M. Kramer, K. Rajurkar, M. Leu, and
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D. Durham of the Division of Design, Manufacture, and Industrial Innovation (DMII) and Dr. J. Larsen Basse of the Surface Engineering and Tribology Program of NSF. In addition, the author thanks Dr. W. Coblenz of DARPA for his interest and support of this work. One of the authors (R.K.) also acknowledges the support from the DOD EPSCoR Program on `Finishing of Advanced Ceramicsa and the MOST Chair for Intelligent Manufacturing. Part of Dr. Yoshino's research was also supported by Grant-in-Aid for Scienti"c Research (No. 09750134) and the Program for Overseas Researchers (No.7-42, 1995) of the Ministry of Education of Japan.
References [1] Puttick KE, Whitmore LC, Chao CL, Gee AE. Transmission electron microscopy of nanomachined silicon crystals. Philosophical Magazine A 1994;69(1):91}103. [2] Yoshino M, Obikawa T, Shirakashi T. Ductile mode cutting of glass. Proceedings of the International Conference on Machining Technology in Asia and Paci"c Regions, Guangzhou, China. South China University of Technology Press, 1993. p. 1}6. [3] Puttick KE, Jeynes C, Rudman M, Gee AE, Chao CL. Surface damage in nanomachined silicon. Semiconductor Science and Technology 1992;7:255}9. [4] Shibata T, Ono A, Kurihara K, Makino E, Ikeda M. Cross-section transmisssion electron microscope observations of diamond-turned single crystal Si surfaces. Applied Physics Letters 1994;65(20):2553}5. [5] Puttick KE, Hosseini MM. Fracture by a pointed indenter on near (1 1 1) Silicon. Journal of Physics D: Applied Physics 1980;13:875}80. [6] Lawn BR, Swain MV. Microfracture beneath point indentations in brittle solids. Journal of Material Science 1975;10:113}22. [7] Lawn BR. Hertzian fracture in single crystals with the diamond structure. Journal of Applied Physics 1968;39:4828. [8] Callahan DL, Morris JC. Extent of phase transformation in silicon hardness indentation. Journal of Material Research 1992;7:1614}7. [9] Morris JC, Callahan DL. Origins of microplasticity in low-load scratching of silicon. Journal of Material Research 1994;7:2907}13. [10] Clarke DR, Kroll MC, Kirchner PD, Cook RF. Amorphization and conductivity of silicon and germanium induced by indentation. Physical Review Letters 1988;60(21):2156. [11] Tanikella BV, Somasekhar AH, Sowers AT, Nemanich RJ, Scattergood RO. Phase transformations during microcutting tests on silicon. Applied Physics Letters 1996;69(19):2870. [12] Puttick KE, Rudman ME, Smith KJ, Frank A, Lindsay K. Single point diamond machining of glass. Proceedings of the Royal Society Series (London) A 1989;426:19}30. [13] Yoshino M, Aoki T, Chandrasekaran N, Shirakashi T, Komanduri R. Finite element simulation of plane strain plastic}elastic indentation on single crystal silicon. International Journal of Mechanical Sciences 2001;43:313}33. [14] Yoshino M, Aoki T, Sugisima T, Shirakashi T. Machinability of hard, brittle materials under high hydrostatic pressure. Proceedings of the Autumn meeting of the Japan Society of Precision Engineering, 1998. p. 258.
Some experiments on the scratching of silicon: In situ scratching inside an SEM and scratching under high external hydrostatic pressures M. Yoshino , T. Aoki , T. Shirakashi, R. Komanduri * Tokyo Institute of Technology, Tokyo, Japan Tokyo Denki University, Tokyo, Japan School of Mechanical and Aerospace Engineering, Oklahoma State University, 218 Engineering North, Stillwater, OK 74078, USA Received 30 April 1999; received in revised form 17 December 1999; accepted 12 January 2000
Abstract To elucidate the mechanisms of material removal in ultra precision machining of silicon involving deformation and fracture, in situ scratching of silicon with a diamond stylus inside a scanning electron microscope (SEM) using a specially designed tribometer and scratching under zero and high (400 MPa) external hydrostatic pressures using a specially designed high-pressure machining apparatus were conducted. The resulting scratches were examined in an SEM to evaluate the in#uence of depth of cut and hydrostatic pressure on the nature of scratches produced (smooth versus fractured surfaces) and the possible brittle}ductile transition. Experimental results indicate that hydrostatic pressure plays an important role in minimizing fracture and producing smooth surfaces. Reasonable agreement of the experimental results with the meso-plasticity FEM simulation of indentation was obtained. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Silicon is the basic substrate material in the manufacture of the chips in the semiconductor industry. Currently, 200 mm diameter silicon wafers are processed routinely to produce the semiconductor chips. However, global competition is forcing industry to further reduce costs in
* Corresponding author. Tel.: 001-405-744-5900; fax: 001-405-744-7873. E-mail address: [email protected] (R. Komanduri). 0020-7403/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 0 - 7 4 0 3 ( 9 9 ) 0 0 0 1 9 - 9
336
M. Yoshino et al. / International Journal of Mechanical Sciences 43 (2001) 335}347
"nishing silicon. It is anticipated that the next millennium would see the end of the 200 mmdiameter silicon wafer era and the dawn of the larger, 300 mm-diameter silicon wafer era. This is based on the history of the semiconductor industry where utilizing larger diameter wafers were found to result in the reduction of manufacturing costs and increase in the production performance signi"cantly. However, in order to produce 300 mm silicon wafers, it is necessary to accomplish much higher precision and better surface integrity than those with a 200 mm silicon wafer. Such requirements may exceed the performance capabilities of the conventional "nishing technology, namely, lapping/polishing practiced today. Consequently, a new machining technology for the "nishing of larger size silicon wafers is anticipated to replace the conventional "nishing technology. A promising technology towards the production of defect free "nishing of silicon is single point diamond turning using a high precision, extremely rigid machine tool. Some researchers have reported the technical feasibility of single point diamond turning of silicon when the size of cut is below a critical value [1]. They termed it `ductile regimea turning indicating plastic deformation of silicon ahead of the tool forming a shear zone followed by the chip formation akin to the machining of a ductile metal where long continuous chips are generated ahead of the tool. They argue that the material behaves in a ductile manner below the critical depth and in a brittle manner above it. Others, such as Yoshino et al. [2] investigated the possibility of a continuous chip formation by a #ow-mode or a `ductile-modea machining of glass and other hard materials, such as ceramics when the size of the cut was below a critical thickness ((1lm). Puttick et al. [3], however, cautioned on the crucial importance of the resulting surface damage. They reported the mean depth of permanent damage to be in the range of 100}400 nm. Shibata et al. [4] conducted diamond turning tests along the [1 1 0] direction on the (0 0 1) plane of a single-crystal silicon at depths of cuts of 100 and 500 nm. Cross-section TEM revealed conversion of the surface layers (&150 nm thick) to an amorphous structure followed by a plastically deformed layer (2}3 lm thick) with numerous dislocations. They also showed evidence of a typical continuous chip by turning silicon at 100 nm cut depth. At the higher depth of cut, namely, 500 nm, material removal was found to be of brittle nature. The interesting aspect of this work is that they used a tool with a !403 rake. Such high negative rakes are hardly used in conventional machining of ductile materials. However, it appears feasible to machine silicon with single crystal diamond only when large negative rake angle are used (!303 and higher). Such large negative rake angles can induce considerable hydrostatic pressure to plastically deform the material directly underneath the tool. Puttick and Hosseini [5] also investigated the initiation of fracture during indentation and scratching on near (1 1 1) silicon surfaces. Cleavage on +1 1 0, planes rather than on +1 1 1, together with a preference for crack propagation in the surface layers were observed. They proposed that +1 1 0, cleavage is initiated by dislocation interactions. There are other researchers who investigated the fracture and brittle nature of silicon [6,7]. The formation of amorphous silicon was also reported in hardness indentation and scratching after the indenter was unloaded [8}10]. Tanikella et al. [11] investigated microcutting of silicon and observed the presence of amorphous silicon within the cutting grooves as well as in the cutting debris located outside of the grooves. Puttick et al. [12] investigated machining of brittle materials including silicon and various glasses, such as soda-lime glass and fused quartz. They showed macrographs of the machined surfaces of fused quartz (Spectrosil) where many areas contained swarf adhering to the sides of the grooves. They also showed numerous ribbons detaching from one side of the grooves only, leaving a striped structure or tearing a ragged foil alongside the original groove or two "laments of the
M. Yoshino et al. / International Journal of Mechanical Sciences 43 (2001) 335}347
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workmaterial peeling o! in opposite directions. Finally, they showed many strips in a state of incipient detachment with loops "xed at both ends. They actually considered that these "laments are generated in the grooves ahead of the tool. These ribbons appear to have been formed not in front of the grooves but on the sides of the grooves as Puttick et al. [12] comment on some of these strips. The delamination and subsequent separation of the "laments led Puttick et al. [12] to conclude that the material on either side of the machined groove is in a state of longitudinal compressive residual stress and subjected to buckling. They also proposed a plastic}elastic peeling e!ect due to the presence of residual stresses. The mechanism of material removal in micromachining of glass, according to Puttick et al. is accomplished by at least two distinct processes, namely, groove formation and subsequent swarf removal, though their relative importance may vary between materials. The fact that the two are not concurrent is particularly noteworthy. It appears that the groove formation is due to indentation and the swarf formation is due to side #ow and subsequent peeling and separation of the "lamentary material during subsequent sliding. To understand the mechanism of material removal in single point ultraprecision machining of silicon, it is necessary to simulate conditions as close to it as possible. Although indentation and scratching process is not exactly equivalent to the machining process, it is commonly used as a "rst step in the modeling of the machining process. In conventional machining, a cutting tool indents into the workmaterial followed by scratching/cutting by the tool. Also, a cylindrical shape of the stylus gives plane strain conditions similar to orthogonal machining. Therefore, the dislocation generation and propagation under plane strain conditions are expected to be similar to the practical scratching/machining process. In the previous investigation [13], meso-plasticity FEM simulation of indentation of single crystal silicon under plane strain conditions was conducted. In this investigation in situ indentation-scratching tests with a diamond stylus inside an SEM using a special tribometer as well as machining under zero and high (400 MPa) external hydrostatic pressure using a specially designed machining apparatus inside a pressure vessel were conducted for a better appreciation of the process.
2. Experiments on scratching of silicon 2.1. In situ scratching inside an SEM using a tribometer Fig. 1(a) is a schematic of a pin-on-disk type tribometer used for the in situ scratching of silicon inside the scanning electron microscope (SEM). The tribometer replaces the normal manually operating rotating stage of the SEM. It is equipped with a small load cell (strain gage-based) to measure the scratch force during the scratch test. Fig. 1(b) shows some details of the load cell. The stylus is mounted on a brass foil on which four strain gages are mounted. From the variation of resistance of these strain gages, both the normal and the tangential forces can be measured independently. On top of the stylus, a small single-crystal diamond pin is located. Fig. 2 is an SEM micrograph of a diamond tip from which the radius of the tip is estimated to be &15 lm. The scratching process conducted inside the SEM was observed in real time and stored using a video recorder. SEM micrographs were also taken at various stages of the scratch process as well as under various scratching conditions.
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Fig. 1. Tribometer used for the in situ scratching of silicon inside the SEM. (b) Detail of the load cell used in the tribometer.
The silicon sample is mounted on the rotating stage of the SEM with a silver conducting cement. The stylus is positioned gently on the workmaterial and the desired normal force is applied by adjusting the sti!ness of a spring attached on the other side of the lever. With the tribometer inside the SEM, the chamber is closed and pumped down to the required vacuum to operate the SEM.
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Fig. 2. SEM micrograph of the diamond pin. The radius of the tip is estimated to be &15 lm.
The position of the stylus is adjusted by the movement of the X}> table from outside the SEM chamber. The rotating stage is driven externally by a motor at a given speed. The motor can be driven in both directions so that the scratching process can be observed both in front of and behind the stylus. The following test conditions were used for in situ scratching inside the SEM : Specimen: single crystal silicon +1 1 1, plane on the surface, Thrust force: 1}3 N, Scratch speed: 265.9 lm/s, Scratch direction: 11 1 02:03,11 1 22:303, Atmosphere: vacuum (10\}10\ Torr), no lubricant. 2.2. Scratching under high hydrostatic pressures: To investigate the e!ect of external hydrostatic pressure on the defect generation in indentation/scratching/machining, a special machining apparatus for conducting scratching under high external hydrostatic pressure (400 MPa) conditions was developed [14]. Fig. 3(a) shows a schematic of the machining apparatus built inside a high hydrostatic pressure vessel. Kerosene was used as the pressurizing #uid. The pressure vessel was made of steel with a design pressure of 500 MPa. The chamber was connected to an external high-pressure pump through the pressure intake. The shaft of the turntable is connected to a variable speed motor. Scratching speeds from 0.8 to 8 mm/s can be obtained with this apparatus. The turntable is supported by an angular contact bearing and leakage around the shaft is prevented using Bridgman seals. Fig. 3(b) shows details of the machining stage. The silicon specimen is attached to the turntable. A diamond stylus
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Fig. 3. (a) Schematic of the experimental setup used in the scratching of silicon under high external hydrostatic pressures: 1. pressure vessel, 2. pressure intake, 3. turntable, 4. angular bearing, 5. Bridgman seal, 6. sleeve, 7. Bridgman seal, 8. lid, and 9. bolts to secure the lid. (b) Details of the scratching apparatus: 10. specimen, 11. tool holder, 12. fulcrum of the tool holder, 13. diamond stylus, 14. spring, 15. spring adjuster.
Fig. 4. SEM micrograph of the diamond stylus. The radius of the tip is estimated to be &17 lm.
is glued to the tool holder with an epoxy bond and is supported by a fulcrum. Fig. 4 is an SEM micrograph of a diamond stylus from which the radius of the tip is estimated to be &17 lm. The scratches formed under di!erent conditions were examined in an SEM to investigate the nature of deformation and fracture.
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3. Results and discussion 3.1. Results of in-situ silicon scratching inside SEM: Figs. 5(a)}(d) are SEM micrographs of the grooves made on the silicon specimens in the 03 scratch direction. Values of the width, w and the depth, d of the grooves are given in the "gure. Depth of the groove, d is calculated using the equation
d"R! R!
w , 2
(1)
Fig. 5. SEM micrographs of the grooves made on the silicon specimens. (a)}(d) Scratch direction is 03. Values of the width, w and depth, d of the grooves are given in the "gures.
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where R is the radius of the pin. It can be seen that smooth grooves can be generated at depths of cut of up to about 0.6 lm after which some cracking can be seen. Also, material plowed to the sides of the groove can be seen. Fig. 6 is the variation of normal force, F with the indentation depth, X showing the e!ect of L normal force on the crack generation by scratching. Solid symbols in the "gure indicate that cracks were generated in the scratching process while open symbols indicate absence of cracks. The critical depth below which no cracks are formed is found to be &0.6 lm. Thus, defect free scratching seems to depend on the normal force (or the depth of the groove). The critical force varies from 1.8 N to 2.0 N while the critical depth of the groove varies from 0.5 to 0.7 lm. As shown in Figs. 5(a)}(c), when the depth of the groove is less than the critical depth, no cracks are formed along the groove; instead many deformed silicon chips are seen. The nature of the chips suggest that silicon may have been deformed plastically. When the depth of the groove is larger than the critical depth [Fig. 5(d)], cracks were generated periodically and broken fragments due to brittle fracture were formed. This phenomena, however, does not seem to depend on the scratching direction based on experimental observations. Fig. 7 shows the relationship between the tangential force and the normal force for 11 1 02 and 11 1 22 directions of scratching. It can be seen that the ratio of tangential to the normal force varies with the rotation angle and the direction of scratching. For the 11 1 02 direction of scratching, the mean friction coe$cient between the pin and the silicon workmaterial is &1.0 (or the friction angle &453) and for the 11 1 22 direction of scratching it is &0.75. 3.2. Results of scratching of silicon under high external hydrostatic pressure: Figs. 8(a) and (b) are SEM micrographs of the grooves made on the silicon specimens at no (or zero) external hydrostatic pressure and at 400 MPa hydrostatic pressure, respectively. They show a smooth surface under high external hydrostatic pressure (400 MPa) and fracture at no (or zero) external hydrostatic pressure. Thus hydrostatic pressure is very e!ective in reducing machining defects, such as microcracks. Fig. 9 shows the relationship between the groove cross-sectional area and the crack ratio. The crack ratio is de"ned as the total length of cracks along unit length of the
Fig. 6. Variation of the normal load with the indentation depth. Solid symbols in the "gure indicate that cracks were generated in the scratching process while open symbols indicate appearance of no cracks. The critical depth below which no cracks are formed is &0.6 lm.
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Fig. 7. Relationship between the tangential force and the normal force for 11 1 02 and 11 1 22 directions of scratching.
Fig. 8. (a) and (b) SEM micrographs of the grooves made on the silicon specimens at zero or no hydrostatic pressure and at 400 MPa hydrostatic pressure.
Fig. 9. Relationship between the groove cross-sectional area and the crack ratio.
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groove. Open circles are for crack ratio under no (or zero) hydrostatic pressure, and solid circles for crack ratio under a hydrostatic pressure of 400 MPa. This "gure shows that the critical value of the groove sectional area, where cracks do not appear, increases by about 10 times under high hydrostatic pressure (400 MPa) than without any external hydrostatic pressure. It can be shown that the origin of these defects is not on the surface but in the interior of the specimen. As shown schematically in Fig. 10(a), if defects in the form of microcracks are present in
Fig. 10. (a) and (b) Schematics indicating the origin of the defects at or near the surface and in the interior, respectively and the in#uence of high hydrostatic pressure on crack propagation or arrest.
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the material, the hydrostatic pressure can prevent crack propagation by closure of these cracks. Thus under the in#uence of high hydrostatic pressure, crack propagation can be arrested and the amount of machining defects can be decreased. On the other hand, if the cracks are present on or near the surface, as illustrated in Fig. 10(b), the pressure medium is forced into the cracks and hydrostatic pressure reaches the tip of the cracks. This hydrostatic pressure balances with the hydrostatic pressure from outside of the specimen, and the e!ect of hydrostatic pressure is cancelled. Hence, the amount of machining defects would not decrease if the defects were attributed to the surface cracks. Therefore, the origin of machining defects must exist in the interior of the workmaterial. Fig. 11 shows variation of defect density during indentation under zero and high (400 MPa) external hydrostatic pressures obtained using the meso-plasticity FEM simulation [13]. It shows that the defect density is reduced to nearly half under the in#uence of high (400 MPa) hydrostatic pressure compared to zero hydrostatic pressure for all values of the indentation depth up to an indentation depth of 2 lm. Also, with increase in the depth of indentation, while the defect density increases rapidly under zero or no hydrostatic pressure, it increases at much lower rate under high (400 MPa) hydrostatic pressure. For example, let us assume the critical defect density, g to be 60 MPa. The critical indentation depth (or critical thickness) for this is &2 lm under zero or no high hydrostatic pressure while it is &5 lm under high (400 MPa) hydrostatic pressure. This shows the e!ect of hydrostatic pressure on critical size of indentation. It, therefore, appears that the generation of defects in the machining of brittle materials, such as silicon are related to the ductile}brittle transition. The machining defects can be attributed not necessarily to the preexisting microcracks in the workmaterial but to the concentration of dislocations generated by plastic deformation under light loads and high hydrostatic pressures immediately underneath the indenter and propagated further into the interior of the workmaterial as indentation progresses. It can be seen that while the trends in the defect structure are somewhat similar for the analytical and the experimental values, there is di!erence in the magnitude. For example, in the experimental value, the critical depth was &0.6 lm while the analytical results indicate a much higher value,
Fig. 11. E!ect of external hydrostatic pressure (400 MPa) on the maximum value of defect density [13].
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namely, &2 lm (Fig. 11). This di!erence can partially be attributed to the fact that only loading (and not the unloading) was considered in this investigation. The unloading could a!ect the critical depth. Also, there are some basic di!erences in the nature of deformation, #ow, and fracture in the indentation and scratching processes.
4. Conclusions 1. Scratching tests were conducted on a single-crystal silicon workmaterial using a pin-on-disctype tribometer inside an SEM and a special scratching apparatus in a high external hydrostatic pressure vessel to study the e!ect of scratch depth and hydrostatic pressures on the nature of scratch generation in silicon. 2. Scratching at various loads indicate brittle fracture above a certain load (&1.8}2 N) and practically no cracking and generation of smooth surfaces below that load. Based on the relationship between the normal load and the indentation depth, the critical depth above which cracks are formed is found to be &0.6 lm. When the depth of the groove is less than the critical depth, no cracks are found along the groove; instead many deformed silicon chips were found suggesting that silicon may have been deformed plastically. When the depth of a groove is larger than the critical depth, cracks are generated periodically and broken fragments were found to come out as a result of brittle fracture. Also, plowing on the sides of the groove can be seen. 3. Scratching tests were conducted on a single-crystal silicon workmaterial using a special machining apparatus built inside a high hydrostatic pressure vessel to study the e!ect of external hydrostatic pressure. Results indicate cracking under zero or no external hydrostatic pressure and practically no cracking and generation of smooth surfaces under high hydrostatic pressure (400 MPa). The critical value of the groove sectional area where cracks do not appear was found to increase by &10 times under high hydrostatic pressure (400 MPa) than without any external hydrostatic pressure. 4. While grooves with smooth surfaces and absence of any fracture can be generated in silicon, the formation mechanism may have no resemblance to the conventional machining with a single point tool, namely, plastic deformation ahead of the tool and formation of long continuous chips. Instead plastic deformation underneath the tool due to high hydrostatic pressure and conversion of silicon from semiconductor to metallic state may be taking place. This is followed by delamination and subsequent separation of the "laments on either side of the machined grooves which are in a state of longitudinal compressive residual stress and subjected to buckling, as proposed by Puttick et al. [12].
Acknowledgements This work was supported by a grant from the US}Japan co-operative program on High E$ciency, Damage-free Finishing of Advanced Ceramics and Glasses (INT-9603002). The authors thank the US National Science Foundation and the Ministry of Education of Japan for support of this work. In particular, one of the authors (R.K.) extends his sincere thanks to Dr. L. Webber of the International Programs of NSF and Drs. L. Matin-Vega, B.M. Kramer, K. Rajurkar, M. Leu, and
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D. Durham of the Division of Design, Manufacture, and Industrial Innovation (DMII) and Dr. J. Larsen Basse of the Surface Engineering and Tribology Program of NSF. In addition, the author thanks Dr. W. Coblenz of DARPA for his interest and support of this work. One of the authors (R.K.) also acknowledges the support from the DOD EPSCoR Program on `Finishing of Advanced Ceramicsa and the MOST Chair for Intelligent Manufacturing. Part of Dr. Yoshino's research was also supported by Grant-in-Aid for Scienti"c Research (No. 09750134) and the Program for Overseas Researchers (No.7-42, 1995) of the Ministry of Education of Japan.
References [1] Puttick KE, Whitmore LC, Chao CL, Gee AE. Transmission electron microscopy of nanomachined silicon crystals. Philosophical Magazine A 1994;69(1):91}103. [2] Yoshino M, Obikawa T, Shirakashi T. Ductile mode cutting of glass. Proceedings of the International Conference on Machining Technology in Asia and Paci"c Regions, Guangzhou, China. South China University of Technology Press, 1993. p. 1}6. [3] Puttick KE, Jeynes C, Rudman M, Gee AE, Chao CL. Surface damage in nanomachined silicon. Semiconductor Science and Technology 1992;7:255}9. [4] Shibata T, Ono A, Kurihara K, Makino E, Ikeda M. Cross-section transmisssion electron microscope observations of diamond-turned single crystal Si surfaces. Applied Physics Letters 1994;65(20):2553}5. [5] Puttick KE, Hosseini MM. Fracture by a pointed indenter on near (1 1 1) Silicon. Journal of Physics D: Applied Physics 1980;13:875}80. [6] Lawn BR, Swain MV. Microfracture beneath point indentations in brittle solids. Journal of Material Science 1975;10:113}22. [7] Lawn BR. Hertzian fracture in single crystals with the diamond structure. Journal of Applied Physics 1968;39:4828. [8] Callahan DL, Morris JC. Extent of phase transformation in silicon hardness indentation. Journal of Material Research 1992;7:1614}7. [9] Morris JC, Callahan DL. Origins of microplasticity in low-load scratching of silicon. Journal of Material Research 1994;7:2907}13. [10] Clarke DR, Kroll MC, Kirchner PD, Cook RF. Amorphization and conductivity of silicon and germanium induced by indentation. Physical Review Letters 1988;60(21):2156. [11] Tanikella BV, Somasekhar AH, Sowers AT, Nemanich RJ, Scattergood RO. Phase transformations during microcutting tests on silicon. Applied Physics Letters 1996;69(19):2870. [12] Puttick KE, Rudman ME, Smith KJ, Frank A, Lindsay K. Single point diamond machining of glass. Proceedings of the Royal Society Series (London) A 1989;426:19}30. [13] Yoshino M, Aoki T, Chandrasekaran N, Shirakashi T, Komanduri R. Finite element simulation of plane strain plastic}elastic indentation on single crystal silicon. International Journal of Mechanical Sciences 2001;43:313}33. [14] Yoshino M, Aoki T, Sugisima T, Shirakashi T. Machinability of hard, brittle materials under high hydrostatic pressure. Proceedings of the Autumn meeting of the Japan Society of Precision Engineering, 1998. p. 258.