1 Introduction

Silicon (Si) is ubiquitous in contemporary technology. It is the second most abundant element in the earth’s crust, existing naturally within various oxygen-rich minerals [1]. Si is an essential ingredient of modern technology because of its wide use in electronic devices [1,2,3,4]. It is a mainstay of semiconductor technology due to the elemental abundance, relatively low costs, ease of doping with other elements, as well as a native oxide passivation layer [1]. Single crystal Si is used to manufacture more than 90% of the semiconductor devices [5]. They are the foundation of the electronics industry, which is the largest industry all over the world. Si wafers are widely used as the substrate of semiconductor devices.

Grinding is one of the most important methods in manufacturing Si wafers and in thinning the device wafers [6,7,8,9,10]. It has both advantages of high accuracy and efficiency to remove the wear layers left on Si wafers by wire sawing from an ingot. The wear layer is the defect on a single crystal, which is induced by grinding. It consists of an amorphous layer at the top and a damaged crystalline layer beneath. Less cost and time for the subsequent chemical mechanical polishing (CMP) is associated with a thinner wear layer. Therefore, it is significant to review the thickness of wear layers produced by grinding. Table 1 lists the thicknesses of wear layers and grinding parameters of diamond grinding and chemical mechanical grinding (CMG) [11,12,13,14,15]. The thicknesses of wear layers are 41,230, 7020, 170–215, and 160–180 nm, which is ground by diamond wheels with mesh sizes of 600, 2000, 3000, and 5000, respectively. Thicknesses of wear layers decrease greatly in one order of magnitude with increasing mesh sizes from 600, 2000, to 3000. However, the thickness of wear layers remains invariable with further increasing of mesh sizes from 3000 to 5000. This explains why the diamond wheel with mesh size of 3000 is the most popular product in ultraprecision grinding of Si wafers. The feed rate of diamond wheels is usually 10 μm/min [11,12,13,14,15], as well as 20 μm/min in diamond grinding with a mesh size of 3000 [13]. To reduce the thickness of wear layers, CMG is proposed in grinding the Si wafers. The thicknesses of wear layers formed by CMG vary from 10 to 35 nm, as listed in Table 1, which is comparable to that of CMP. Nevertheless, soft abrasives are used in CMG, rather than the hardest diamond abrasives. This results in a feed rate of only 1 μm/min [12,13,14]. Considering the deformation of soft abrasives during grinding, the actual material removal rate of CMG is also similar to that of CMP. Therefor, it is a challenge to obtain nanoscale wear layers for diamond grinding in high efficiency at a feed rate of 10 μm/min, which is highly desirable for semiconductor devices and the electronics industry to save cost and time for subsequent CMP processes. CMP is the most expensive process in manufacturing Si wafers, in terms of the operation, transportation, storage, and final treatment of chemical reagents.

Table 1 Thicknesses of wear layers and grinding parameters of diamond grinding and CMG [11,12,13,14,15]
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In this study, a novel approach of mechanical chemical grinding (MCG) is proposed. Two types of diamond wheel are developed. Nanoscale wear layers are obtained in diamond grinding with high efficiency. The grinding and wear mechanisms are investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectra, and high resolution transmission electron microscopy (HRTEM) equipped with selected area electron diffraction (SAED).

2 Experimental Details

Commercially available 6-inch Si wafers (Grinm Advanced Materials Co., Ltd.) were used as specimens. One surface of a Si wafer was polished by CMP, which is fixed on the vacuum chuck. The other side of a Si wafer was a corrosive surface, which was used for MCG. Two types of diamond wheel were developed, and are designated as M1 and C2 with magnesia and ceria, respectively, as listed in Table 2. The bond used in the two wheels was zinc oxide (ZnO). Diamond abrasives, bond, and additive powders were mixed uniformly at room temperature and then pressed into segments by isostatic compaction. A segment was 22 mm long, 5 mm wide, and 10 mm high. The segments were sintered and vitrified completely at temperatures varying from 780 to 800 °C. Forty-four segments were distributed uniformly and glued in a notch at the periphery of an aluminum (Al) alloy plate. The diameter of the Al alloy plate was 350 mm. The concentration of diamond in a wheel was 200, which is a volume content, equivalent to a volume fraction of 50%. The mesh size of two diamond wheels was 20,000.

Table 2 Description of the two developed diamond wheels
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MCG was performed on an ultraprecision grinder (Okamoto, VG401 MKII, Japan), as shown in Fig. 1 and especially in Fig. 1c. The grinder could feed vertically only, guaranteeing the grinding accuracy with two air spindles. Taking into account the truing efficiency and quality, two truing methods were employed, as listed in Table 3. A diamond wheel was firstly trued by a cast iron plate with free abrasives of silicon carbide (SiC), followed by an alumina plate with deionized water as coolant. After truing, MCG was conducted on the corrosive surfaces of Si wafers. The grinding parameters are listed in Table 3. The grinding speed of wheels was 40.3 m/s, and the feed rates were set at 5, 10, 15, and 20 μm/min, respectively.

Fig. 1
figure 1

Photographs of M1 (a), its ground Si wafer at a feed rate of 5 μm/min (b), and MCG on an ultraprecision grinder (c), and surface roughness (d) induced by two diamond wheels on ground Si wafers as a function of feed rate

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Table 3 Grinding parameters of the two developed diamond wheels
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The vitrified diamond segments and ground Si wafers were characterized using a field emission SEM (FEI, Quanta 200 FEG, the Netherlands) equipped with EDS and measured by XRD (Bruker AXS D8 Discover, Germany) with copper radiation, XPS (PHI Quantera II, Japan), and Raman spectra (Renishaw inVia Reflex, UK) excited by a green laser with a wavelength of 532 nm. TEM samples were prepared firstly by a dimple grinder (Gatan 656, USA), then polished by a disc grinder manually (Gatan 623, USA), and finally polished using a precision ion polishing system (Gatan 695, USA). TEM samples were characterized using an FEI Tecnai F20 at low magnification operated at 200 kV, and an FEI Titan G2 60-300 aberration corrected HRTEM at high magnification operated at 300 kV. Hardness and elastic modulus were measured by a nanomechanical tester (TI 950 TriboIndenter®, Hysitron, Minneapolis, MN, USA) at peak loads of 4, 5, and 6 N, respectively. Surface roughness R a, root mean square (rms) and peak-to-valley (PV) values were measured by a non-contact precision surface profilometer (Zygo, NewView 5022, USA). The area was 71 × 53 μm2 for each measurement of surface roughness.

3 Results

Figure 1 shows the photographs of M1, its ground Si wafer and MCG on an ultraprecision grinder, and surface roughness induced by two diamond wheels on ground Si wafers. A vitrified diamond wheel of M1 is developed, as illustrated in Fig. 1a. A ground Si water of M1 looks bright and like a mirror, pictured in Fig. 1b. M1 is mounted on an ultraprecision grinder and a Si wafer is fixed on vacuum chuck, as shown in Fig. 1c. Surface roughness R a and PV values of C2 are obvious lower than those of M1, as drawn in Fig. 1d. Surface roughness induced by two diamond wheels is listed in Table 4. The surface roughness R a produced by M1 varies from 1.24 to 1.79 nm, with increasing feed rates from 5 to 20 μm/min, corresponding to the PV values changing from 10.8 to 13.5 nm.

Table 4 Surface roughness and calculated undeformed chip thickness induced by the two developed diamond wheels
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Figure 2 illustrates the SEM images of M2 and its EDS, XRD, XPS, and Raman spectra. The surface of M2 is porous and looks uniform, as shown in Fig. 2a, b). The C element is the most up to 54.86 wt% (Fig. 2c), which is consistent with the highest peak of diamond in Fig. 2d. The weight percentages of Mg, Zn, and Si elements are 14.55, 12.2, and 1.35% (Fig. 2c), respectively. Diamond, MgO, ZnO, zinc carbonate (ZnCO3), SiO2, and SiC are confirmed by XRD in Fig. 2d. ZnO, MgO, and SiC are identified by XPS in Fig. 2e at 1021.7, 88 and 100.9 eV, respectively [16, 17]. A sharp diamond peak is present at 1332 cm−1 [18] (Fig. 2f), and a broad amorphous peak centered at 1430 cm−1 is the characteristics of amorphous carbon (a–C), deriving from the diamond abrasives [19, 20].

Fig. 2
figure 2

SEM images of M2 at low (a) and high (b) magnifications and its EDS (c), XRD (d), XPS (e), and Raman (f) spectra in (a)

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Figure 3 pictures SEM images of ground Si wafers of M1 at feed rates of 5 and 20 μm/min. All the ground surfaces have wear tracks, without cracks, indicating ductile grinding characteristics. Moreover, the ground surface in Fig. 3d is smeared, meaning the alleviative grinding.

Fig. 3
figure 3

SEM images of ground Si wafers of M1 at feed rates of 5 (a), (b) and 20 (c), (d) μm/min at low (a), (c) and high (b), (d) magnifications

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Figure 4 shows the representative EDS, XRD, XPS, and Raman spectra on ground Si wafers of M1 at a feed rate of 5 μm/min. A Si peak is found by EDS in Fig. 4a. SiO2 peaks are confirmed by XRD in Fig. 4b. Si and SiO x peaks are identified by XPS spectra in Fig. 4c at 98.4 and 102.2 eV, respectively [16, 17]. A sharp Si peak is determined by Raman spectrum in Fig. 4d at 520 cm−1 [21]. Hereby, Si, SiO2 , and SiOx are identified by EDS, XRD, XPS, and Raman spectra, verifying the clean grinding of M1 and without contaminants generated by M1.

Fig. 4
figure 4

EDS (a), XRD (b), XPS (c), and Raman spectra (d) on ground Si wafers of M1 at a feed rate of 5 μm/min

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Figure 5 illustrates the cross-sectional TEM images of a ground Si wafer by M1 at a feed rate of 5 μm/min. The wear layer is 94 nm in thickness, consisting of an amorphous layer at the top, followed by a damage crystalline layer underneath (Fig. 5a). It is interesting that the nanoscale wear layer induced by M1 is obtained. The SAED pattern illustrates the pristine Si-I phase, absent of high pressure phases, as seen in the inset of Fig. 5a. In Fig. 5b, the distances between crystalline planes are different, indicating the formation of crystallites.

Fig. 5
figure 5

Cross-sectional TEM images of a ground Si wafer by M1 at a feed rate of 5 μm/min at low (a) and high (b) magnifications. Inset showing its corresponding SAED pattern marked in a black circle in (a)

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Figure 6 shows the truing of C2 mounted on an ultraprecision grinder and its ground Si wafer. The developed diamond wheel C2 is trued by a green alumina plate, which is fixed by vacuum chuck of an ultraprecision grinder, as diagramed in Fig. 6a. A ground Si wafer of C2 looks bright and like a mirror, as illustrated in Fig. 6b.

Fig. 6
figure 6

Photographs for truing of C2 (a) mounted on an ultraprecision grinder and its ground Si wafer (b)

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Figure 7 shows the SEM images of C2 and its EDS, XRD, XPS, and Raman spectra in (a). The surface of vitrified diamond wheel C2 is porous, as illustrated in Fig. 7a, b. C2 consists of C, O, Mg, Si, Ce, Zn elements with weight percentages of 13.98, 3.17, 1.65, 1.99, 64.09, 15.12%, respectively, as listed by EDS in Fig. 7c. Sharp peaks of CO2 are identified by XRD in Fig. 7d, whose intensities are stronger than those of diamond. Only CeO2 and diamond crystalline peaks are found in XRD of C2, exhibiting the perfect vitrified effect of C2, compared with M1. ZnO peaks are determined by XPS in Fig. 7e at 10.3, 89, 140, and 1022 eV for Zn 3d, Zn 3p3/2, Zn 3s, and Zn 2p3/2, respectively [17]. SiC peaks are identified at 101.5 and 151.3 eV for Si 2p3/2 and Si 2 s, respectively. CeO2 peak is found at 882.9 eV for Ce 3d5/2 [17]. Sharp CeO2 and diamond peaks are found by Raman spectra in Fig. 7f at 466 [22] and 1332 [18] cm−1 respectively, and the intensity of the former is stronger than that of the latter. This is consistent with the results of XRD in Fig. 7d. A broad peak centered at 1430 cm−1 corresponds to the a–C, which is derived from the diamond abrasives [19, 20].

Fig. 7
figure 7

SEM images of C2 at low (a) and high (b) magnifications and its EDS (c), XRD (d), XPS (e), and Raman spectra (f) in (a)

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Figure 8 pictures the SEM images of Si wafers ground by C2 at feed rates of 5 and 20 μm/min. There are no cracks and sharp wear tracks on all the ground Si wafers, indicating the characteristics of mild and ductile grinding. This is in a good agreement with the ground Si wafers of M1 in Fig. 3. The surface roughness R a and PV values ground by C2 keeps basically constants at 1 and 9.8 nm, respectively, with increasing feed rates from 5 to 20 μm/min, as listed in Table 4 and illustrated in Fig. 1d. The surface roughness R a and PV values are 0.95 and 10.1 nm, respectively, on Si wafers ground by C2 at a feed rate of 20 μm/min, indicating ultra-smooth surfaces obtained in high efficiency.

Fig. 8
figure 8

SEM images of Si wafers ground by C2 at feed rates of 5 (a), (b) and 20 (c), (d) μm/min at low (a), (c) and high (b), (d) magnifications

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Figure 9 diagrams the representative EDS, XRD, XPS, and Raman spectra on Si wafers ground by C2 at a feed rate of 20 μm/min. Si and SiO2 are ascertained by EDS and XRD, respectively, corresponding to Fig. 9a, b. Si and SiO x are identified at 98.6 and 102.3 eV, respectively, in Fig. 9c, which corresponds to Si 2p [16, 17]. A sharp Si peak is confirmed by Raman spectra at 520 cm−1 in Fig. 9d [21]. Si, SiO2 and SiO x are found by EDS, XRD, XPS, and Raman spectra, displaying the clean grinding, without contaminants from the diamond wheel of C2. This agrees well with the experimental results of M1 in Fig. 4.

Fig. 9
figure 9

EDS (a), XRD (b), XPS (c), and Raman spectra (d) on ground Si wafers of C2 at a feed rate of 20 μm/min

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Figure 10 illustrates the cross-sectional TEM images of a ground Si wafer by C2 at a feed rate of 20 μm/min. The SAED pattern exhibits the pristine Si-I phase, without high pressure phases, as found in the inset of Fig. 10a. The wear layer of 56 nm in thickness consists of an amorphous layer at the top and a damage crystalline layer beneath, as shown in Fig. 10a, which is less than one third that ground by a conventional diamond wheel with a mesh size of 3000 [12,13,14]. Crystal lattice of damage crystalline layer is shown in Fig. 10b, and the distances between crystalline places are different, indicating the generation of crystallites.

Fig. 10
figure 10

Cross-sectional TEM images of a ground Si wafer by C2 at a feed rate of 20 μm/min at low (a) and high (b) magnifications. Inset showing its corresponding SAED pattern marked in a black circle in (a)

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4 Discussion

The maximum undeformed chip thickness, h m can be used to characterize the grinding conditions [23,24,25]. h m is significant for the grinding performance and processing. Usually, the smaller the h m , the thinner of wear layers ground by diamond wheels, and the lower surface roughness. It is calculated [24, 25],

$$h_{m} = \frac{{E_{1} }}{{E_{2} }}\left( {\frac{4}{{rC^{1.5} }}\frac{{v_{f} }}{{v_{s} }}} \right)^{0.5}$$
(1)

where, E 1 and E 2 are the elastic moduli of diamond wheels and specimens, respectively, r is the ratio of width to thickness of an undeformed chip, C is the quantity of surface active grains per unit area, and v f and v s are the feed rates of the diamond wheel and grinding speed, respectively. Figure 11 draws the hardness and elastic modulus of the two developed diamond wheels as a function of load. The hardness of M1 and C2 is approximately 178 and 600 MPa respectively, as shown in Fig. 11a. This is because of the porous structure on the surfaces of two vitrified diamond wheels. The elastic moduli of M1 and C2 are 30.8 and 38.1 GPa, respectively, as illustrated in Fig. 11b. r is 1.49 [25]. C is expressed [24, 25],

$$C = \frac{4f}{{d_{\text{g}}^{2} \left( {\frac{4\pi }{3v}} \right)^{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0pt} 3}}} }}$$
(2)

where f is the ratio of active grains to the total ones on the surface, d g is the equivalent diameter of grains, and v is the volume fraction of diamond grains. f is 0.5 in grinding [26]. d g is presented [27],

$$d_{\text{g}} \left( {\text{mm}} \right) = \frac{15.2}{M}$$
(3)

where M is the mesh size of a diamond wheel. d g is 760 nm for the two diamond wheels with a mesh size of 20,000. h m is listed in Table 4. It increases monotonously from 0.49 to 0.98 nm and from 0.6 to 1.21 nm for M1 and C2, respectively, with increasing feed rates from 5 to 20 μm/min. All values of h m for M1 and C2 are less than 1.3 nm, which is very small, contributing greatly to the nanoscale wear layers on Si wafers, as illustrated in Figs. 5 and 10, as well as for lower surface roughness listed in Table 4. Nevertheless, the surface roughness and wear layer ground by C2 are much smaller than those of M1, at the same grinding parameters and mesh sizes of grains for the two diamond wheels. This is attributed to the different MCG effects of M1 and C2 on Si wafers.

Fig. 11
figure 11

Hardness (a) and elastic modulus (b) of the two developed diamond wheels as a function of load

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M1 consists of diamond, ZnO and MgO in majority, and a bit of SiC and SiO2, as illustrated in Fig. 2. ZnCO3 is formed during sintering, benefiting for the generation of porous structure in the vitrified diamond wheel. C2 includes mainly diamond, ZnO and CeO2, and a few SiC, as pictured in Fig. 7. ZnO is the bond in both M1 and C2. In developing a new diamond wheel, two types of ceramics are appropriate during sintering to control the temperature. Two ceramics are insensitive for a small variation of temperature, facilitating the formation of vitrified diamond wheel. Thus, both M1 and C2 consist of two main ceramics, in addition to diamond grains. Some additives of ceramics are beneficial for the production of porous structure in the two vitrified diamond wheels, due to the different volume shrinkage of ceramics during sintering [23]. MgO and CeO2 are responsible for the MCG effects in M1 and C2, respectively.

ZnO is used to decrease the dissolution rate of the vitrified block made of diamond wheels, increasing their lifetime [28]. During sintering, non-bridging oxygen is created within the vitrified bond of ZnO, strengthening the interfacial bonding between diamond grains and bond [29]. Furthermore, the oxidation resistance of diamond grains is increased under the protection of ZnO, enhancing the bending strength of vitrified blocks composed of two diamond wheels [30]. Consequently, ZnO is selected as the bond of two developed diamond wheels, for obtaining good grinding performance.

SiC is used in both diamond wheels of M1 and C2. It exhibits high thermal conductivity [31], low thermal expansion, low fracture toughness, chemical inertness at elevated temperature, high hardness and wear resistance, and low density [23, 32]. In M1 and C2, the diamond grains are very small, and their equivalent diameter is 760 nm. For this reason, SiC participates in grinding, especially when diamond grains are worn and pulled out. Moreover, it can take away the grinding heat immediately induced by grinding, because of its high thermal conductivity. This is because SiC has a ratio of 9:1 of covalent to ionic bonding [33]. On the other hand, SiC and SiO2 can bond well [34]. Hence, SiC and SiO2 are both added in M1. Furthermore, SiO2 promotes the uniform distribution of vitrified amorphous phase, decreases the melting temperature and reduces the nucleation of crystalline phases during sintering [35].

CeO2 and MgO are stable in air. However, a Si wafer is exposed in air, as explained in the following equation,

$${\text{Si}} + {\text{O}}_{2} \to {\text{SiO}}_{2}$$
(4)

During grinding, the air is insufficient at the grinding zone, as shown in Eq. 5,

$$2{\text{Si}} + x{\text{O}}_{2} \to 2{\text{SiO}}_{x}$$
(5)

CeO2 and MgO do not react with SiO2 at room temperature. They can react with SiO2 under stress induced by grinding firstly, followed by the united effects of mechanical force and heat generated by friction during grinding. Moreover, diamond grinding happens in both M1 and C2 to remove the material of workpiece in high efficiency, which is different from CMG in a low material removal rate. Wherefore, a novel approach of MCG is proposed. The reactions between MgO and Si or SiO2 are as follows,

$${\text{Si}} + {\text{O}}_{2} + {\text{MgO}} \to {\text{MgSiO}}_{3}$$
(6)
$${\text{MgO}} + {\text{SiO}}_{2} \to {\text{MgSiO}}_{3}$$
(7)

These two equations are activated by the mechanical force firstly, and then by the combined effects of mechanical force and grinding heat induced by grinding. The MCG of M1 alleviates the aggressive grinding taken place in traditional diamond grinding, and wild ground surfaces are observed in Fig. 3. Under the MCG effect, the smeared surface ground by M1 is found in Fig. 3c, and a wear layer of 94 nm in thickness is produced by grinding in Fig. 5a. Thus, the nanoscale wear layer is obtained.

Compared with MgO, CeO2 has unique mechanical and chemical characteristics. For instance, it has high thermal stability, in terms of a melting point up to 2400 °C [36]. This means that CeO2 is in a perfect crystalline phase during sintering, as illustrated in Fig. 7(d). As an ultrathin SiO2 layer is formed on a Si wafer exposed to air, grinding will remove the SiO2 layer first. MCG begins from the mechanical force first. Under grinding, a mechanical force is applied on the Si wafers, which makes the O bonds of SiO2 broken. This triggers the reduction reactions of CeO2 from Ce4+ to Ce3+ under the coherent effects of mechanical force and grinding heat. Four equations are suggested in air and deionized water during grinding of C2 [37, 38],

$$4{\text{Ce}}^{4 + } + {\text{Si}}^{0} \to 4{\text{Ce}}^{3 + } + {\text{Si}}^{4 + }$$
(8)
$${\text{Si}} + {\text{SiO}}_{2} + 4{\text{CeO}}_{2} \to 2{\text{Ce}}_{2} {\text{O}}_{3} + 2{\text{SiO}}_{2}$$
(9)
$$4{\text{SiO}}_{x} + 3{\text{O}}_{2} + 4{\text{CeO}}_{2} \to 2{\text{Ce}}_{2} {\text{O}}_{3} + 2x{\text{O}}_{2} + 4{\text{SiO}}_{2}$$
(10)
$$4{\text{CeO}}_{2} + 6{\text{H}}_{2} {\text{O}} \to 4{\text{Ce}}\left( {\text{OH}} \right)_{3} + {\text{O}}_{2}$$
(11)

The reduction reactions lead to the transfer of electrons from the p-orbital of O atoms to the f-orbital of Ce atoms, forming the Si–O–Ce bonding with the dangling bonds of O atoms [39]. With the formation of Si–O–Ce bonding, SiO2 lump is pulled out and removed from the SiO2 layer, under the combined effects of mechanical force and heat induced by grinding [39]. Si4+ and Ce3+ ions are easily removed at the forms of silicates and cerium hydroxide in deionized water, respectively. The reduction reactions of CeO2 cause C2 to obtain a wear layer of 56 nm in thickness, as illustrated in Fig. 10a, as well as lower surface roughness compared to M1, as pictured in Fig. 1d. Nanoscale wear layer produced by C2 in high efficiency is consistent with previous reports, in which CeO2 obtains the highest material removal rate in abrasive machining, compared with Al2O3, TiO2, Y2O3, ZrO2, SnO2, Cr2O3, and La2O3 [40]. MCG of C2 starts from the mechanical force applied by grinding, followed by the united effects of mechanical force and grinding heat.

5 Conclusions

In summary, two types of diamond wheel are developed to perform MCG. M1 consists of diamond abrasives, ZnO and MgO in majority, and a bit of SiC and SiO2. C2 includes diamond abrasives, ZnO and CeO2 mainly, and a few SiC. The addition of CeO2 makes C2 exhibit only two crystalline phases of CeO2 and diamond, indicating perfect vitrified effect. Nanoscale wear layers are obtained on ground Si wafers by the two developed diamond wheels. Surface roughness R a and PV values are stable on ground Si wafers by C2, with increasing feed rates from 5 to 20 μm/min. All the surface roughness R a and PV values induced by C2 are lower than those of M1. Nanoscale wear layers in high efficiency are obtained on Si wafers in MCG of C2, which is attributed to the reduction reactions of CeO2 under the combined effects of mechanical force and grinding heat. MCG has great potential applications in the fields of semiconductors, microelectronics, and optoelectronics.