- Email: [email protected]
Microstructure studies of the grinding damage in monocrystalline silicon wafers
RARE METALS Vol. 26, No. 1, Feb 2 0 0 7 , ~13 . E-mail: [email protected]
Available online at www.sciencedirect.com i i -
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Microstructure studies of the grinding damage in monocrystalline silicon wafers ZHANG Yinxia, KNVG Renke, GUO Dongrning, and JIN Zhuji Key hboratory for Precision and Non-traditionalMachining Technology of Chinese Ministry of Education, Dalian University of Technology, Dalian 116024. China
(Recehed 2005-12-23)
Abstract: The depth and nature of the subsurface damage in a silicon wafer will limit the performance of IC components. Damage microstructures of the silicon wafers ground by the #325, MOO, and #2OOO grinding wheels was analyzed. The results show that many microcracks, fractures, and dislocation rosettes appear in the surface and subsurface of the wafer ground by the #325 grinding wheel. No obvious microstructure change exists. The aniorphous layer with a thichness of about 100 nm, microcracks, high density dislocations, and polycrystalhe silicon are observed in the subsurface of the wafer ground by the #600 grinding wheel. For the wafer ground by the #2000 grinding wheel. an amorphous layer of about 30 nm thickness, a polycrystallie silicon layer, a few dislocations, and an elastic deformation layer exist. In general, with the decrease in grit size, the material removal mode changes from micro-fracture mode to ductile mode gradually. Key words: silicon wafers; grinding; subsurface damage; microstructure [This study wasfinuncially suppoaed by the National Natural Science Foundation of China in Major Project ProgrcJm (No. 50390061) and the National Science Fund for Distinguished Young Scholars (No. 50325518).]
1. Introduction The monocrystalline silicon wafer, as an important substrate of the integrated circuit, is widely used in IC manufacturing. Various processes are needed to transfer a silicon crystal ingot into wafers. Silicon crystallizes in the diamond lattice, with covalent bonding, ensuring an extremely stable spatial arrangement of the Si atoms in the monocrystal. It is a brittle material. Most processes can induce mechanical damage. The depth and nature of the subsurface damage will limit the performance and life of IC components [l-21. Grinding has the potential to replace conventional lapping because of its high efficiency and low damage. Wafer rotation grinding, as an important grinding technology, is widely used in manufacturing and back thinning of the silicon wafers. However, the grinding damage has important influences on the surface quality and the output Corresponding author: ZHANG Yinxia
of the wafer in subsequent processes 131. To satisfy the rising demand for the high integrity of wafer surface, it is essential to control the subwrface damage of ground wafers. The study of the depth and nature of the subsurface also benefits the understanding of the damage mechanism. Results from microindentation, microscratching, and procbessing (slicing, dicing, single diamond turning, lapping, and grinding) have demonstrated that diamond-cubic silicon can transform to the denser metallic p-tn structure under certain conditions at room temperature. Upon release of pressure, phase transformations occur. According to some reported results, it has been asserted that the origin of ductile behavior is attributed to the pressure-induced phase transformations [4-61. To identify the grinding mechanism, it is essential to analyLe the nature of damage using corresponding measurement methods. The subsurface damage is difficult to measure because the
E-mail: [email protected]
RARE METALS, Vol. 26, No. I , Feb 2007
14
damage depth is less, especially for the fine ground wafers. Although there are many methods to assess the subsurface damage, such as step etching, angle polishing, and Raman spectroscopy, the transmission electron microscope (TEM) observation is an efficient method to evaluate the detailed information of the damage 17-81, The present study is designed to analyze subsurface microstructure change of the ground silicon wafers by means of TEM on cross-section view and plane-view samples. The Raman microspectroscopy is also used to study the phase transformation. The results of the wafers ground by different grinding wheels are analyzed.
2. Experimental 2.1. Grinding experiment The grinding experiment was conducted on an ultra-precision grinder (VG4OlMK U). The grinding wheels applied were resin-bond diamond cup wheels with mesh sizes of #325, #600, and #2000. During grinding, the silicon wafer was held on a porous ceramic vacuum chuck, and the grinding wheel and the wafer were rotated about their own axes of rotation simultaneously. The grinding wheel had an axial down feed and de-ionized water was used as cooling fluid. The silicon wafers used in this study were 200 mm in diameter, which were produced by the Czochralski technique and oriented at (100) plane. The silicon wafers were damage-free because they were lapped and etched for an adequate amount of time. Thus, the damage detected thereafter must have been caused by grinding. Table 1 summarizes the grinding parameters.
2.2. Sample preparation and TEM observation The <110> cross-section TEM samples were fust scribed using an automatic abrasive disk dicing saw and cleaved into narrow pieces with dimensions of 2 mm x 6 mm along the (1 10) plane. The cross-section was straight, flat, and perpendicular to the ground surface. The ground surfaces of two pieces were glued face to face with M-Bond 600 adhesive as shown in Fig. l(a). The cross-section of
the sample was then lapped and polished till the sample was 0.6 mm thick and then the other side was lapped and polished until the sample approached less than 20 pm in thickness as shown in Fig. l(b). Table 1. Wafer rotation grinding conditions Method
Wafer rotation grinding
Silicon wafers
(100) CZ Si wafers ($200 mm)
Wheels
$350 mm resin-bonded diamond cup wheels (mesh size: #325, #600, #2000)
Whcel rotational speed, n, / (r.min-')
2 100, 1800,2400
Chuck rotational speed, n, / I~r.min-')
300,250, 120
Down feed rate of wheel 90, 100, 10 spindle,f/ (pmmin-')
DI water (20°C)
Coolant
e& (110)
(' 100) (a)
(b)
Fig. 1. Illustration of cross-section TEM sample preparation.
Finally, the sample was ion milled on the Model 691 precision ion polishing system (PIPS) to make a strong specimen with an electron-transparent central area. For the plane view samples, they were prepared by mechanical thinning and then the side opposite to the ground surface was ion milled. The lapping and polishing processes were realized by applying silicon carbide abrasive papers with different mesh sizes and abrasive sluny, respectively. All the preparation processes were carried out at roorn temperature. After preparation, the samples were observed using TEM (Philips TECNAI G' 20). The electron microscopes were operated at an accelerating voltage of 200 kV. In addition, the ground surfaces were also observed using an atomic force microscope (AFM XE-100) and Raman microscope (JY-HR 800).
Zhang Y.X. et al., Microstructure studies of the grinding damage in monocrystallinesilicon wafers
3. Results and discussion The Liurface topographies of the ground silicon wafers measured by AFM are shown in Fig. 2. The corresponding surface roughnesses of the wafers ground by the #325, %Oo, and #2000 grinding wheels ;re 79. 31, and 13 nm, respectively. The wafer ground by the #325 grinding wheel is full of mi-
15
crocracks and ground in a brittle fracture mode. But the ductile removal mode prevails in the wafer ground by the #2000 grinding wheel. For the wafer ground by the #600 grinding wheel, some material is removed in ductile mode. In general, with the decrease in grit size, the surface becomes microcrack-free and smooth. The mesh size has an important effect on the material removal mode.
Fig. 2. .ikM 3D images (5 px 5 p) of wafer surfaces ground by the #325 (a), #600 (b), and #2OlMl (c) grinding wheels.
To characterize the microstructure of the near surface .egion in the ground silicon wafers, cross sectional analyses were carried out. Fig. 3 is the optical croIs sectional image of the wafer ground by the # 3 3 grinding wheel. Many microcracks and fractures appear in the top region. The subsurface damage is severe. The crack configurations are complicated. The depth of subsurface cracks changes from 1 pm to 10 pm. Figs. 4 (a) and 4(b) show cr8)ss sectional TEM images and the corresponding selected area electron diffraction patterns (SAEDP)of the sample ground by the #325 grinding wheel. The electron diffraction pattern of the near surface 1 egion shows that only the diamond-cubic structura silicon (Si-I) exisls. Below the crack layer. there are many dislocation rosettes. The subsurface damage jepth is about 15 pm. The material is removed ir fracture mode.
Fig. 3. CrmT sectional optical image of the wafer ground by the #325 grinding wheel.
The subsurface structure of the wafer ground by the #600 grinding wheel presents two distinct regions as shown in Figs. 5(a) and 5(b). The top region is an amorphous structure without any crystalline particles. The crystalline silicon exists underneath the amorphous layer. In the interface the crystal phase which immersed in an amorphous medium is present. The amorphous layer is about 100 nm in thickness and the thickness is not uniform in the ground surface. There are some microcracks, polycrystalline silicon. and two obvious dislocation slip systems immediately beneath the ground surface. The existence of the amorphous structure and dislocations can be further identified by the ' E M analysir of the plane view sample as shown in Fig. 6. The crystalline silicon beneath the amorphous layer are Si-I, Si-rII (bc8-body-centered cubic structure), and Si-XI1 (r8-rhombohedra1 structure). The results are confirmed by the Raman spectroscopy studies shown in Fig. 7. The results %howthe Si-111 phase (164 cm-', 384 cm-', 436 cm?), Si-XI1 (351 cm?), and amorphous silicon (a-Si: the broad peaks around 150 cm-' and 470 cm-') are observed in the wafer ground by the #600 and #2OOO grinding wheels. But almost no phase transformation occurs in the wafer ground by the #325 grinding wheel. On
16
RARE METALS, Vol. 26, No. 7, Feb 2007
Cross sectional TEM images of the wafer ground by the #325 grinding wheel: (a) fractures; (b)microcracks and dislocations.
Fig. 4.
Amorphous layer
Fig. 5. Cross sectional TEM images of the wafer ground by the #600 grinding wheel: (a) microcracks;(b) amorphow layer and dislocatiom.
7-
I
I
0
Fig. 0. Plane view TEM micrograph of the wafer surface ground by the #6OU grinding wheel.
Fig. 7. Ranian spectra of wafer surfaces ground by the #325 (a), #600 (bj, and #2800 (c) grinding wheels.
the surface ground by the #600 grinding wheel, metastahle phases such as Si-nI and Si-XI1 are found in large quantities compared with that observed on the surfa:e ground by the #2000 grinding wheel. The
presence of the Si-III and Si-XI1 confirms that the Si momentarily exists in the metallic phase Si-II because the transformation horn Sj-1 to Si-WSi-XIU a-Si is possible only by first transforming into me-
Zhung Y.X. et ul., Microstructure studies of the grinding damage in monocrystalline silicon wafers
tallic phase Si-II [9-101. The Si-11 is beneficial for the grinding process because it is ductile and can be removed without damaging the underlying crystal silicon. The subsurface damage depth of the wafer ground by the #600 grinding wheel is about 3 pm. Fig. 8 is the cross sectional "EM image and the electron diffraction patterns of the wafer sample ground by the #2000 grinding wheel. Fig. 8(b) is a HRTEM image taken from the material in the region (I) illustrated in Fig. 8(a). There is an amorphous layer about 30 nm in thickness in the top region of the subsurface. Underneath the amorphous layer the polycrystalline layer and a few dislocations exiqt. This result is further identified by the diffraction analysis of the plane view specimen shown in Fig. 9. The electron diffraction patterns illustrated in Figs. 8(c) and 8(d) are Si-I with the [I 101 zone axis and Si-III with [110] zone axis respectively. Most crystals are Si-I. The HRTEM image shows the diffcrent crystal structure of the interface between the amorphous phase and the crystalline silicon. The (11), (ID), and (IV) marked in Fig. 8(b) are crystalline silicon
I7
and the (I) marked in Fig. 8(b) is the amorphous phase. The interface of the amorphous and crystal phase is rough and some crystalline silicon exists inside the amorphous phase. The subsurface damage depth is about 1.3 ym. Grinding is a two-body abrasion process and the silicon wafer surface is subject to contact wlth numerous abrasive grits simultaneously. The nurnerous particles on the wheel act as mriny small tips. They produce different contact pressures and create different depths of cut because of their respective shapes and sizes. The different grinding wheels have different grit sizes. Thus, the grinding depth can be controlled through choosing the corresponding grinding wheel and grinding parameter. With the increase of the ginding depth and the grinding force, both the density and penetration depth of dislocations in the wafer subsurface increase. When the dislocations penetrate deep, with complex interactions, severe stress concentration occurs and microcracks appear. The microcracks are determined by the dislocation structure in the subsurface. For the
Amorphous layer
/ Surface
(a) Cross sectional TEM image of the waler ground by the #2OOO grinding wheel; (b) HRTEM image taken from the material in the region I illustrated in (a); (c) and (d) electron diffraction pattern5 of the Si-I and Si-nI.
Fig. 8.
RARE METALS, Vol. 26, No. 1,Feb 2007
18
wafer ground by the #325 grinding wheel, the grinding stress exceeds the fractured strength, so many microcrdcks initiate and the material is removed by the brittle fracture mode. But the metallic phase transformation occurs during grinding by the #600 and #2000 grinding wheels. Some materials are removed by plastic deformation. Therefore, the material removal mechanism can be divided into two modes, the brittle fracture mode and the ductile mode. The ductile removal mechanism is related to the phase transformation. In the present study the brittle fracture removal mode is realized by the #325 grinding wheel. For the #600 grinding wheel, although metallic phase transformation occurs, the brittle fracture mode prevails. The ductile removal mode prevails in the wafer ground by the #2000 grinding wheel.
Fig. 9. Plane View HRTEM micrograph of the wafer surface ground by the #2000 grinding wheel.
4. Conclusions (1) For the wafer ground by the #325 grinding wheel, many microcracks and fractures appear in the surface and subsurface of the wafer. The material is removed entirely using the brittle fracture mode. (2) The microcracks, two obvious dislocation slip systems, and polycrystalline silicon are obscrved in the subsurface of the wafer ground by the #600 grinding wheel. For the wafer ground by the #2000 grinding wheel, no obvious microcracks exist. Only a few arrays of dislocations exist. (3) Phase transformations occur during grinding with the #600 and #2000 grinding wheels. The in-
terface between amorphous and polycrystalline silicon is rough and some crystalline silicon exists inside the amorphous medium. The amorphous layer, about 100 nm and 30 nm in thicknesses exists in the wafers ground by the #600 and #2000 grinding wheels, respectively. With the decrease in grit size, the amorphous layer becomes thinner and the micro defects become fewer. (4) In general, with the decrease in grit size, the material removal mode changes from brittle fracture mode to ductile mode gradually.
References Tonshoff H.K., Schmieden W.V., and Inasalu I., 4brasive machining of silicon, Ann. CIRP, 1990, 39 i(2): 621. ILucca D.A., Brinksmeier E., and Gwh G. Progress in assessing surface and subsurface intcgrity. Ann. CIRP, 1998,47 (2):669. Pei Z.J. and Strasbaugh A,, Fine grinding of silicon wafers: designed experiment, Znt. J. M ~ i c h . Tools .Munu$, 2002,42: 395. Bradly J.E., Williams J.S., and Wong-hung J., Me'chanicaldeformation in silicon by micro- indentation, .I. Muter. Rex, 2001, 16 (5): 1500. Kunz R.R.. Clark H.R., Nitishin P.M., and Kothschild M., High resolution studies oT crystalline damage induced by lapping and single-point diamond machining of Si (IOO), J. Muter. Kes., 1996, 11( 5 ) : 1228. Puttick K.H., Whitmore L.C., Chao C.L.. md Gee A.E., Transmission electron microscopy of nanomachined silicon crystal. Phil. Mug A.. 1994,69: 91.
Zhang L.C., and Zamdi I.. Effect of ultra-precision grinding on the microstructural change in silicon monocrystals, J. Muter. Process. Technol.. 1998. 84: 149. Zhang L.C., and Zanidi I., Towards a deeper understanding of plastic deformation in moncxystalline silicon, Int. J. Mech. Sci., 2001, 43: 1985. Gogotsi Y., Beak C., and Kirscht F., Raman microspectroscopy study of processing-induced phase transformations and residual stress in silicon. Semicoid. Sci. Techrid., 1999, 14: 936. [lo] Kailer A., Gogotsi Y.G., and Nickel K Ci., Phase transformations of silicon caused by contact loading, J. Appl. Phys, 1997.81: 3057.
Available online at www.sciencedirect.com i i -
-1Z ScienceDirect
Microstructure studies of the grinding damage in monocrystalline silicon wafers ZHANG Yinxia, KNVG Renke, GUO Dongrning, and JIN Zhuji Key hboratory for Precision and Non-traditionalMachining Technology of Chinese Ministry of Education, Dalian University of Technology, Dalian 116024. China
(Recehed 2005-12-23)
Abstract: The depth and nature of the subsurface damage in a silicon wafer will limit the performance of IC components. Damage microstructures of the silicon wafers ground by the #325, MOO, and #2OOO grinding wheels was analyzed. The results show that many microcracks, fractures, and dislocation rosettes appear in the surface and subsurface of the wafer ground by the #325 grinding wheel. No obvious microstructure change exists. The aniorphous layer with a thichness of about 100 nm, microcracks, high density dislocations, and polycrystalhe silicon are observed in the subsurface of the wafer ground by the #600 grinding wheel. For the wafer ground by the #2000 grinding wheel. an amorphous layer of about 30 nm thickness, a polycrystallie silicon layer, a few dislocations, and an elastic deformation layer exist. In general, with the decrease in grit size, the material removal mode changes from micro-fracture mode to ductile mode gradually. Key words: silicon wafers; grinding; subsurface damage; microstructure [This study wasfinuncially suppoaed by the National Natural Science Foundation of China in Major Project ProgrcJm (No. 50390061) and the National Science Fund for Distinguished Young Scholars (No. 50325518).]
1. Introduction The monocrystalline silicon wafer, as an important substrate of the integrated circuit, is widely used in IC manufacturing. Various processes are needed to transfer a silicon crystal ingot into wafers. Silicon crystallizes in the diamond lattice, with covalent bonding, ensuring an extremely stable spatial arrangement of the Si atoms in the monocrystal. It is a brittle material. Most processes can induce mechanical damage. The depth and nature of the subsurface damage will limit the performance and life of IC components [l-21. Grinding has the potential to replace conventional lapping because of its high efficiency and low damage. Wafer rotation grinding, as an important grinding technology, is widely used in manufacturing and back thinning of the silicon wafers. However, the grinding damage has important influences on the surface quality and the output Corresponding author: ZHANG Yinxia
of the wafer in subsequent processes 131. To satisfy the rising demand for the high integrity of wafer surface, it is essential to control the subwrface damage of ground wafers. The study of the depth and nature of the subsurface also benefits the understanding of the damage mechanism. Results from microindentation, microscratching, and procbessing (slicing, dicing, single diamond turning, lapping, and grinding) have demonstrated that diamond-cubic silicon can transform to the denser metallic p-tn structure under certain conditions at room temperature. Upon release of pressure, phase transformations occur. According to some reported results, it has been asserted that the origin of ductile behavior is attributed to the pressure-induced phase transformations [4-61. To identify the grinding mechanism, it is essential to analyLe the nature of damage using corresponding measurement methods. The subsurface damage is difficult to measure because the
E-mail: [email protected]
RARE METALS, Vol. 26, No. I , Feb 2007
14
damage depth is less, especially for the fine ground wafers. Although there are many methods to assess the subsurface damage, such as step etching, angle polishing, and Raman spectroscopy, the transmission electron microscope (TEM) observation is an efficient method to evaluate the detailed information of the damage 17-81, The present study is designed to analyze subsurface microstructure change of the ground silicon wafers by means of TEM on cross-section view and plane-view samples. The Raman microspectroscopy is also used to study the phase transformation. The results of the wafers ground by different grinding wheels are analyzed.
2. Experimental 2.1. Grinding experiment The grinding experiment was conducted on an ultra-precision grinder (VG4OlMK U). The grinding wheels applied were resin-bond diamond cup wheels with mesh sizes of #325, #600, and #2000. During grinding, the silicon wafer was held on a porous ceramic vacuum chuck, and the grinding wheel and the wafer were rotated about their own axes of rotation simultaneously. The grinding wheel had an axial down feed and de-ionized water was used as cooling fluid. The silicon wafers used in this study were 200 mm in diameter, which were produced by the Czochralski technique and oriented at (100) plane. The silicon wafers were damage-free because they were lapped and etched for an adequate amount of time. Thus, the damage detected thereafter must have been caused by grinding. Table 1 summarizes the grinding parameters.
2.2. Sample preparation and TEM observation The <110> cross-section TEM samples were fust scribed using an automatic abrasive disk dicing saw and cleaved into narrow pieces with dimensions of 2 mm x 6 mm along the (1 10) plane. The cross-section was straight, flat, and perpendicular to the ground surface. The ground surfaces of two pieces were glued face to face with M-Bond 600 adhesive as shown in Fig. l(a). The cross-section of
the sample was then lapped and polished till the sample was 0.6 mm thick and then the other side was lapped and polished until the sample approached less than 20 pm in thickness as shown in Fig. l(b). Table 1. Wafer rotation grinding conditions Method
Wafer rotation grinding
Silicon wafers
(100) CZ Si wafers ($200 mm)
Wheels
$350 mm resin-bonded diamond cup wheels (mesh size: #325, #600, #2000)
Whcel rotational speed, n, / (r.min-')
2 100, 1800,2400
Chuck rotational speed, n, / I~r.min-')
300,250, 120
Down feed rate of wheel 90, 100, 10 spindle,f/ (pmmin-')
DI water (20°C)
Coolant
e& (110)
(' 100) (a)
(b)
Fig. 1. Illustration of cross-section TEM sample preparation.
Finally, the sample was ion milled on the Model 691 precision ion polishing system (PIPS) to make a strong specimen with an electron-transparent central area. For the plane view samples, they were prepared by mechanical thinning and then the side opposite to the ground surface was ion milled. The lapping and polishing processes were realized by applying silicon carbide abrasive papers with different mesh sizes and abrasive sluny, respectively. All the preparation processes were carried out at roorn temperature. After preparation, the samples were observed using TEM (Philips TECNAI G' 20). The electron microscopes were operated at an accelerating voltage of 200 kV. In addition, the ground surfaces were also observed using an atomic force microscope (AFM XE-100) and Raman microscope (JY-HR 800).
Zhang Y.X. et al., Microstructure studies of the grinding damage in monocrystallinesilicon wafers
3. Results and discussion The Liurface topographies of the ground silicon wafers measured by AFM are shown in Fig. 2. The corresponding surface roughnesses of the wafers ground by the #325, %Oo, and #2000 grinding wheels ;re 79. 31, and 13 nm, respectively. The wafer ground by the #325 grinding wheel is full of mi-
15
crocracks and ground in a brittle fracture mode. But the ductile removal mode prevails in the wafer ground by the #2000 grinding wheel. For the wafer ground by the #600 grinding wheel, some material is removed in ductile mode. In general, with the decrease in grit size, the surface becomes microcrack-free and smooth. The mesh size has an important effect on the material removal mode.
Fig. 2. .ikM 3D images (5 px 5 p) of wafer surfaces ground by the #325 (a), #600 (b), and #2OlMl (c) grinding wheels.
To characterize the microstructure of the near surface .egion in the ground silicon wafers, cross sectional analyses were carried out. Fig. 3 is the optical croIs sectional image of the wafer ground by the # 3 3 grinding wheel. Many microcracks and fractures appear in the top region. The subsurface damage is severe. The crack configurations are complicated. The depth of subsurface cracks changes from 1 pm to 10 pm. Figs. 4 (a) and 4(b) show cr8)ss sectional TEM images and the corresponding selected area electron diffraction patterns (SAEDP)of the sample ground by the #325 grinding wheel. The electron diffraction pattern of the near surface 1 egion shows that only the diamond-cubic structura silicon (Si-I) exisls. Below the crack layer. there are many dislocation rosettes. The subsurface damage jepth is about 15 pm. The material is removed ir fracture mode.
Fig. 3. CrmT sectional optical image of the wafer ground by the #325 grinding wheel.
The subsurface structure of the wafer ground by the #600 grinding wheel presents two distinct regions as shown in Figs. 5(a) and 5(b). The top region is an amorphous structure without any crystalline particles. The crystalline silicon exists underneath the amorphous layer. In the interface the crystal phase which immersed in an amorphous medium is present. The amorphous layer is about 100 nm in thickness and the thickness is not uniform in the ground surface. There are some microcracks, polycrystalline silicon. and two obvious dislocation slip systems immediately beneath the ground surface. The existence of the amorphous structure and dislocations can be further identified by the ' E M analysir of the plane view sample as shown in Fig. 6. The crystalline silicon beneath the amorphous layer are Si-I, Si-rII (bc8-body-centered cubic structure), and Si-XI1 (r8-rhombohedra1 structure). The results are confirmed by the Raman spectroscopy studies shown in Fig. 7. The results %howthe Si-111 phase (164 cm-', 384 cm-', 436 cm?), Si-XI1 (351 cm?), and amorphous silicon (a-Si: the broad peaks around 150 cm-' and 470 cm-') are observed in the wafer ground by the #600 and #2OOO grinding wheels. But almost no phase transformation occurs in the wafer ground by the #325 grinding wheel. On
16
RARE METALS, Vol. 26, No. 7, Feb 2007
Cross sectional TEM images of the wafer ground by the #325 grinding wheel: (a) fractures; (b)microcracks and dislocations.
Fig. 4.
Amorphous layer
Fig. 5. Cross sectional TEM images of the wafer ground by the #600 grinding wheel: (a) microcracks;(b) amorphow layer and dislocatiom.
7-
I
I
0
Fig. 0. Plane view TEM micrograph of the wafer surface ground by the #6OU grinding wheel.
Fig. 7. Ranian spectra of wafer surfaces ground by the #325 (a), #600 (bj, and #2800 (c) grinding wheels.
the surface ground by the #600 grinding wheel, metastahle phases such as Si-nI and Si-XI1 are found in large quantities compared with that observed on the surfa:e ground by the #2000 grinding wheel. The
presence of the Si-III and Si-XI1 confirms that the Si momentarily exists in the metallic phase Si-II because the transformation horn Sj-1 to Si-WSi-XIU a-Si is possible only by first transforming into me-
Zhung Y.X. et ul., Microstructure studies of the grinding damage in monocrystalline silicon wafers
tallic phase Si-II [9-101. The Si-11 is beneficial for the grinding process because it is ductile and can be removed without damaging the underlying crystal silicon. The subsurface damage depth of the wafer ground by the #600 grinding wheel is about 3 pm. Fig. 8 is the cross sectional "EM image and the electron diffraction patterns of the wafer sample ground by the #2000 grinding wheel. Fig. 8(b) is a HRTEM image taken from the material in the region (I) illustrated in Fig. 8(a). There is an amorphous layer about 30 nm in thickness in the top region of the subsurface. Underneath the amorphous layer the polycrystalline layer and a few dislocations exiqt. This result is further identified by the diffraction analysis of the plane view specimen shown in Fig. 9. The electron diffraction patterns illustrated in Figs. 8(c) and 8(d) are Si-I with the [I 101 zone axis and Si-III with [110] zone axis respectively. Most crystals are Si-I. The HRTEM image shows the diffcrent crystal structure of the interface between the amorphous phase and the crystalline silicon. The (11), (ID), and (IV) marked in Fig. 8(b) are crystalline silicon
I7
and the (I) marked in Fig. 8(b) is the amorphous phase. The interface of the amorphous and crystal phase is rough and some crystalline silicon exists inside the amorphous phase. The subsurface damage depth is about 1.3 ym. Grinding is a two-body abrasion process and the silicon wafer surface is subject to contact wlth numerous abrasive grits simultaneously. The nurnerous particles on the wheel act as mriny small tips. They produce different contact pressures and create different depths of cut because of their respective shapes and sizes. The different grinding wheels have different grit sizes. Thus, the grinding depth can be controlled through choosing the corresponding grinding wheel and grinding parameter. With the increase of the ginding depth and the grinding force, both the density and penetration depth of dislocations in the wafer subsurface increase. When the dislocations penetrate deep, with complex interactions, severe stress concentration occurs and microcracks appear. The microcracks are determined by the dislocation structure in the subsurface. For the
Amorphous layer
/ Surface
(a) Cross sectional TEM image of the waler ground by the #2OOO grinding wheel; (b) HRTEM image taken from the material in the region I illustrated in (a); (c) and (d) electron diffraction pattern5 of the Si-I and Si-nI.
Fig. 8.
RARE METALS, Vol. 26, No. 1,Feb 2007
18
wafer ground by the #325 grinding wheel, the grinding stress exceeds the fractured strength, so many microcrdcks initiate and the material is removed by the brittle fracture mode. But the metallic phase transformation occurs during grinding by the #600 and #2000 grinding wheels. Some materials are removed by plastic deformation. Therefore, the material removal mechanism can be divided into two modes, the brittle fracture mode and the ductile mode. The ductile removal mechanism is related to the phase transformation. In the present study the brittle fracture removal mode is realized by the #325 grinding wheel. For the #600 grinding wheel, although metallic phase transformation occurs, the brittle fracture mode prevails. The ductile removal mode prevails in the wafer ground by the #2000 grinding wheel.
Fig. 9. Plane View HRTEM micrograph of the wafer surface ground by the #2000 grinding wheel.
4. Conclusions (1) For the wafer ground by the #325 grinding wheel, many microcracks and fractures appear in the surface and subsurface of the wafer. The material is removed entirely using the brittle fracture mode. (2) The microcracks, two obvious dislocation slip systems, and polycrystalline silicon are obscrved in the subsurface of the wafer ground by the #600 grinding wheel. For the wafer ground by the #2000 grinding wheel, no obvious microcracks exist. Only a few arrays of dislocations exist. (3) Phase transformations occur during grinding with the #600 and #2000 grinding wheels. The in-
terface between amorphous and polycrystalline silicon is rough and some crystalline silicon exists inside the amorphous medium. The amorphous layer, about 100 nm and 30 nm in thicknesses exists in the wafers ground by the #600 and #2000 grinding wheels, respectively. With the decrease in grit size, the amorphous layer becomes thinner and the micro defects become fewer. (4) In general, with the decrease in grit size, the material removal mode changes from brittle fracture mode to ductile mode gradually.
References Tonshoff H.K., Schmieden W.V., and Inasalu I., 4brasive machining of silicon, Ann. CIRP, 1990, 39 i(2): 621. ILucca D.A., Brinksmeier E., and Gwh G. Progress in assessing surface and subsurface intcgrity. Ann. CIRP, 1998,47 (2):669. Pei Z.J. and Strasbaugh A,, Fine grinding of silicon wafers: designed experiment, Znt. J. M ~ i c h . Tools .Munu$, 2002,42: 395. Bradly J.E., Williams J.S., and Wong-hung J., Me'chanicaldeformation in silicon by micro- indentation, .I. Muter. Rex, 2001, 16 (5): 1500. Kunz R.R.. Clark H.R., Nitishin P.M., and Kothschild M., High resolution studies oT crystalline damage induced by lapping and single-point diamond machining of Si (IOO), J. Muter. Kes., 1996, 11( 5 ) : 1228. Puttick K.H., Whitmore L.C., Chao C.L.. md Gee A.E., Transmission electron microscopy of nanomachined silicon crystal. Phil. Mug A.. 1994,69: 91.
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