Corresponding Author: M. Konneh, Department of Manufacturing and Materials Engineering, International Islamic University Malaysia, E-mail: mkonneh@iiu.edu.my

International Journal of Science Engineering and Technology Vol. 1, No. 3, 2008 ISSN: 1985-3785 Available online at: www.ijset.org © 2008 ILRAM Publisher

Diamond Grits Effect on the Morphology of Ground Hard-Brittle Silicon M. Konneh1, E. Y. T. Adesta1 and S. Izman2

1 Department of Manufacturing & Materials Engineering, International Islamic University Malaysia, P. O. Box 10, 50728 Kuala Lumpur, Malaysia E-mails: mkonneh@iiu.edu.my, izman@fkm.utm.my 1 Department of Manufacturing & Materials Engineering, International Islamic University Malaysia, P. O. Box 10, 50728 Kuala Lumpur, Malaysia 2 Department of Manufacturing and Industrial Engineering Universiti Tehnologi Malaysia, 81310 Johor, Malaysia Abstract Silicon like any hard and brittle material is well known for its low machinability unless it is machined under ductile mode condition. Selection of appropriate machining parameters would result in production of fine streaks on the machined work-piece surface. Such a precision machining technique is beneficial to wafer labs and chip-making industries as surface streaks are essential for reducing polishing time. This paper deals with an experimental investigation on surface quality in terms of the morphology of ground silicon surface when 5mm-diameter resin bonded diamond wheels were used to machine IC silicon chips. The process factors involve the machining conditions such as speed, feed rate and depth of cut. A specially modified machine tool that has an air driven low powered high-speed attachment was used for the machining. Results indicate partial-ductile streaks as well as some fractures and pits generated at the ground silicon surfaces. KEYWORDS: Hard-brittle silicon, partial ductile-mode grinding, morphology, surface-ductile streaks. 1. Introduction Silicon like any hard and brittle material is known for its low machinability due to its high hardness and low fracture toughness unless it is machined precisely under ductile mode condition. The phenomenon whereby a brittle material behaves like a ductile material when it is machined is referred to as ductile mode machining. Thus the surface morphology of the machined hard-brittle material will composed of fine ductile or partial-ductile streaks. Namba [1] categorized ground surfaces of a brittle material into three i.e. fracture surface, ductile and fracture (partial-ductile) surface, and ductile surface, Figure 1.

Mono-crystalline silicon with (100) plane as the major surface is the most commonly used semiconductor substrate material for the production of microchips. The single crystal silicon is not only a dominant substrate material for the fabrication of micro-electro and micro-mechanical components such as microelectromechanical-systems (MEMS) but also an important infrared optical material Wang et al. [2]. Sometimes IC chips need to have pockets machined on them for failure analysis. In the case of MEMS structures are usually developed by focused ion beam Machining (FIB), an

expensive process that also produces considerable heat but extremely low forces.

Despite the significant importance of silicon

in, especially, electronic industries its machining poses considerable difficulty, Tonshoff et al [3]. Conventional grinding process induces surface damage and this results in strength degradation. Even a small depth of grinding involves a high

(a) (b)

(c)

Figure 1. Namba’s [1] classification of ground surfaces of NbF1 optical glass: (a) Fractured mode ground surface, (b) Ductile and fracture (partial-ductile) surface and (c) Ductile mode ground surface

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specific grinding energy and generates a high grinding temperature. Enhanced grinding temperature tends to promote fracture and cracks. However, because of the high hardness of silicon materials, grinding with a diamond wheel is generally performed as a finishing process. High speeds are possible with diamond tools because of very low frictional forces as well as high heat conductivity tool characteristics that tend to provide relatively low operating temperatures at the cutting edge Gorczyca [4], Balson [5], Ramesh [6]. The work that best describes ductile mode machining is Blackley and Scattergood's [7] and Taniguchi's [8] does likewise with precision engineering. The work of Blackley and Scattergood and others [7, 9] revealed quite unambiguously how surface defects can occur and how they can be avoided at least theoretically. The analysis of the model developed by Blackley and Scattergood is .not easily understood. However careful examination and its interpretation by considering other papers 10, 11] shows how well it explains the formation of a chip whose bulk consists of fractured segments but whose underside is smooth like one would get when machining ductile copper under conventional conditions. Further research work of Venkatesh et. al [12], have demonstrated that under certain controlled conditions, it is possible to machine brittle materials using single or multi-point diamond tools so that material is removed by plastic flow, leaving a crack-free surface. During precise abrasive machining operation, high mechanical loading and excessive heat, which can cause functional defects, must be avoided. Accomplishing such criteria requires gentle, delicate and surgical machining operation. In this investigation, the machining technique is designed to avoid the use of FIB machining that is not only expensive, but produces heat during micro-melting of silicon structures. The process factors considered are speed, feed rate and depth of cut. A specially modified machine tool that has an air driven low powered high-speed jig grinding attachment was used for the machining. Resin bonded diamond pins (wheels) were mainly used because of their good grit protrusion, relatively low cost, and safety at high speeds Zygo surface analyser was used to measure surface roughness and topography of machined surfaces. Optical and Scanning Electron Microscopy techniques were employed to examine surface morphology of the machined specimens. Results indicate formation of partial-ductile streaks as well as some fractures and pits on the ground silicon surfaces. and surface roughness Ra as low as 60 nm. 2. Experimental details 2.1 Work and tool materials

The work-piece materials that were precision micro ground are IC silicon chips (Figure 2(a)), obtained from a chip-making industry, Tool materials are WINTER made resin bonded diamond grinding pins (Figure 2(b)) with specifications 1A1 W-5-6-15 D91 K-888 RY C100 and 1A1 W-5-6-15 D126 K-888 RY C100. 2.2 Experimental equipment and instruments The following equipment and instruments were used during experimentation:

A 3-axis MAHO MH 500 E CNC Vertical Milling Centre with Motor horsepower: 4.5 KW. Work spindle speed, directly programmable: 20 – 5000 rpm. Linear traverse measuring system resolution is 1m.

Ultra-precision High-speed Jig Grinder (NSK PLANET 1500).

Zygo surface analyser. A Carl Zeiss Stemi 2000-c Optical

Microscopes for examining tool surfaces. Scanning Electron Microscope for

examining of machined surfaces. 2.3 Cutting conditions The machining trial conditions are feed rate: 10 min/min, 20 mm/min, 30 mm/min and depth of cut: = 25 µm (kept constant). Each trial run was repeated three times. Water-oil based coolant with ratio 50:1 and coolant flow rate of 10 litres per minute were used. 2.4 Methodology The machining set-up for grinding of thin IC silicon chips is shown in Figure 3. A ground, flat tool steel platform was screwed onto the machine bed and a locally fabricated fixture was fixed on it. The flatness of the fixture after mounting it on the platform was checked with a 1 µm dial indicator and maintained below 5 µm. The main spindle of the 3-axis MAHO CNC vertical-spindle milling machine was replaced by an ultra-precision high-speed jig grinder (NSK Planet 1500) and air supplied to the air motor from an air supply kit. The air supply was maintained at 4 kgf/cm2 (0.4 MPa), giving 100, 000 + 10% rpm, based on the speed

(a) (b)

Figure 2 (a) IC silicon chip work-piece (b) a 5 mm-diameter resin-bonded grinding pin used during experimentation.

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tests conducted with a non-contact digital tachometer.

For a trial run, an IC silicon chip was gently secured in the fixture. The bottom surface of a resin bonded grinding pin was checked on a Zeiss optical microscope for any of contamination, and then dressed, and its bottom surface checked prior to the machining trial run. The work-piece (IC chip) was flattened and flatness of the machined silicon chip surface was checked to accuracy below 5 m prior to a trial run. An experimental trial condition was then tried out. Surface texture and morphology measurements were done on the Zygo surface analyzer. The ground IC silicon chip surface was cleaned using ultrasonic technique, dried, and optical, scanning electron microscopic images of the surfaces were taken. 3 Results and discussion

The results of micro-grinding IC silicon chips with the 91 µm and 126 µm grits of resin-bonded diamond grinding pins are presented in the SEM micrographs of surfaces in Figs 4 and 5 respectively. In all cases, the surfaces consist of partial-ductile streaks and fractures. It is observed from the figures that streaks are evident at conditions where the depths of cut are relatively higher than the feed rates by some factor. These characteristics are depicted in Figures 4(a) and 5(a). This behaviour agrees with Blakeley and Scattergood’s [7, 9], which explains how ductile streaks are generated when turning brittle materials. The SEM images of the surfaces ground with the 91 m grit reveal considerable areas where ploughing action (plastic flow) of abrasives has taken place particularly at the 10 mm/min feed and 25 m depth, while at feeds higher than this, are a combination of plastic deformation and brittle fracture. Jackson [13] categorized the mechanisms that contribute to the wear of grinding wheels into four: abrasive wear (formation of wear flat on the surface of abrasive grains), fracture of abrasive grains due to mechanical and thermal shock load, fracture of

bond bridges and fracture at the interface between abrasive grain and bond-bridge , Fig.6. His work has revealed that grain fracture is the most important mechanism of material removal from perfectly sharp abrasive wheels during grinding. Fracture occurs as a consequence of tensile stresses induced into the abrasive grains by grinding forces to which they are subjected. The existence of relatively low tensile stresses in the bond bridges, or the abrasive grains themselves may cause failure by fracture to occur.

Fixture

Resin-bonded grinding pin

High-speed jig grinder attachment

Figure 3: The figure illustrating experimental setup forprecision micro-grinding of IC silicon chips

Material pullout Ductile streak

(a) (b)

(c)

(a) Feed rate = 10 mm/min, depth of cut = 25 µm (b) Feed rate = 20 mm/min, depth of cut = 25 µm

(c) Feed rate = 30 mm/min, depth of cut = 25 µm

Figure 4: SEM images of IC silicon dies ground with 91 m resin bonded grinding pins. Almost all surfaces consist of fractures and partial-ductile streaks. However, surfaces (a) and (b) show predominantly fairly fine streaks and less fracture.

(a) (b)

(c)

(a) Feed rate = 10 mm/min, depth of cut = 25 µm (b) Feed rate = 20 mm/min, depth of cut = 25 µm

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Grain fracture is equivalent to breaking a piece of stone into two or more pieces in order to expose new sharp edges. Grains are usually expected to fracture or fragment at a moderate rate to continuously produce new sharp cutting edges during grinding. A grain-workpiece combination with high arrtitious wear and low friability dull grains and develop a large wear flat. Grinding then

becomes ineffective, and surface damages such as burning are likely to occur. The inconsistency shown in results at the higher feed rates can be attributed to fracture of grains from the grinding pin surface as observed on 91 µm and 126 grits after a trial condition, Fig.7 although other factors generally affecting roughness cannot be ruled out.

4 Conclusions Surfaces ground with the small grit 91 m

(compared to the 126 m) exhibit considerable amount of partial-ductile streaks though fractures are evident in the predominantly streaked rejoin. Such generated surfaces are vital to chip-making industries for easing polishing. The fairly neat

spacing of the ductile streaks generated by the 91 m grit at the lowest feed rate and higher depth of cut indicates how a finer feed, a high concentration of diamonds, and faceting of diamond grits could bring about ductile streaks.

It has been found that the performance of the 91 m grit of the resin-bonded diamond grinding

(c) Feed rate = 30 mm/min, depth of cut = 25 µm

Figure 5: SEM images of IC silicon dies ground with 126 m resin bonded grinding pins. Surfaces except (b) show predominantly material pullout and fractured regions which are undesirable.

Figure 6: Jackson’s (2002) classification of grinding wheel wear mechanisms: A. Abrasive wear (formation of wear flat on the surface of abrasive grains). B. Abrasive grain fracture. C. Bond-bridge fracture. D. Interfacial grain-bond fracture

(a) Diamond grain

(b) Fractured grain

Figure 8: 3-D topography and roughness values of IC silicon dies ground with 91 m resin bonded diamond grinding pins. Surfaces clearly show grinding marks caused by the cutting actions of the grits attached to the bottom surfaces of the

Figure 7: Grinding wheel (pin) bottom surface showing grain and grain fractures after a grinding pass using (a) 91 µm and (b) 126 µm grit.

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pins is better than the 126 m grit pins with respect to surface morphology of the work-pieces that have been investigated. Experimental results reveal low roughness values on the ground IC chip surfaces. The low roughness values encourage further investigations using finer grits, even much smaller than 91 m. 5 Acknowledgements The authors express their sincere thanks to the Research Management Centre of the International Islamic University Malaysia for funding this project. References [1] Y. Namba, M. Abe, Ultra-precision grinding of

optical glasses to produce super-smooth surfaces, Ann. CIRP 42 (1) (1993) 417420.

[2] X. Wang, G.D. Hong, J. Zhang, B.L. Lim, H.Q. Gong, Precision patterning of diamond Films for MEMS Application, Int. Conf. on Precision Engineering, Singapore, (2000) 562-567.

[3] H.K. Tonshoff, W. Schmiedon, I. Inasaki, W. Konig, G. Spur, (1990), Abrassive machining of silicon, Ann. CIRP 39 (2) (1990) 621-635.

[4] F. E. Gorczyca, Application of metal cutting theory, Industrial Press, Inc. (1987).

[5] P. Balson, R. Puna, Diamond wheel selection criteria for grinding advanced engineering ceramics, SME EM91-196 (1991) 1-20.

[6] R. Ramesh, K. Ramesk, S.H. Yeo, S. Gowri, L. Zhou, Experimental Evaluation of Super High-Speed Grinding of Ceramics, Inter. J. Adv. Manuf. Technol 17 (2001) 87-92.

[7] S. Blakeley, R.O. Scattergood, Mechanics of Material Removal in Diamond Turning, ASPE Annual Meeting, Rochester, NY, (1990) 68-71.

[8] N. Taniguchi, Current Status in and Future Trends of Ultraprecision Machining and Ultrafine Materials Processing, Ann. CRIP 32 (2) (1983) 573-582.

[9] P.N. Blake, Ductile regime diamond turning of germanium and Silicon, North Carolina State University, U.S.A. PhD Thesis (1988).

[10] V.C. Venkatesh, I. Inasaki, H.K. Tonshoff, T. Nakagawa, I.D. Marinescu, I.D., (1995), Observations on Polishing and Ultra Precision Machining of Semiconductor Substrate Materials, Ann. 44 (2) (1995) 611-618.

[11] Z. Zhong, V.C. Venkatesh, Generation of parabolic and toroidal surfaces on Si and Si-based compounds using diamond cup grinding wheels, Ann. CIRP 43 (1) (1994).

[12] V.C. Venkatesh, S.S. Izman., T.T. Mon, M. Konneh, (2004) “Precision grinding of hard and brittle materials”, Pertanika J. Sci. Technol. 12 (2) (2004) 113-126.

[13] Jackson, M.J., (2002). Wear of perfectly sharp abrasive grinding wheels, SME Technical

Paper, MR02-157