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Effects of pressure and slurry on removal mechanism during the chemical mechanical polishing of quartz glass using ReaxFF MD
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Effects of pressure and slurry on removal mechanism during the chemical mechanical polishing of quartz glass using ReaxFF MD ⁎
Xiaoguang Guo, Song Yuan , Junxin Huang, Chong Chen, Renke Kang, Zhuji Jin, Dongming Guo Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Quartz glass CMP ReaxFF MD Removal mechanism Aqueous H2O2 Pressure
Reactive force field molecular dynamics (ReaxFF MD) simulation was employed to study the interfacial interaction during the Chemical mechanical polishing (CMP) process of quartz glass. Results indicated that dissociation and adsorption of H2O occurred on the quartz glass surface in pure H2O, and the dissociation of H2O could hydroxylate quartz glass surface to form SieOH bonds. In addition, it was found that the surface hydroxylation degree is higher in aqueous H2O2. The formation of SieOeSi bonds plays a key role in the CMP process. Dehydrogenation and dehydroxylation are two ways to form interface bridge bonds. The interface bridge bonds between abrasive and substrate could transmit mechanical force with the movement of abrasive. Interface pressure affects the number and forms of atoms removed. With the increment of pressure, the removal form gradually changes from single removal mode to chain removal mode. The effect of aqueous H2O2 on the removal process was also studied by adding different amount of OH group on the quartz glass surface. This work helps us reveal the removal mechanism in the CMP process of quartz glass from an atomic perspective.
1. Introduction
Shen N et al. [8] reported the mechanics of polishing particles near the elastic-plastic load boundary of optical glass by AFM. They also proposed that the surface densification of different optical glasses was different during the CMP process. Molecular dynamics (MD) simulation could explain the mechanism from the atomic perspective and reveal the details of particle interaction in the CMP process. Now, simulation technologies such as tight-binding quantum chemical molecular dynamics (TBQC-MD) [1,9,10] as well as reactive force field (ReaxFF) [11–17] have been applied in MD simulation successfully. Kubo et al. [10,18] performed the SiO2 CMP process with CeO2 abrasive by TBQCMD, indicating that mechanical force induced by CeO2 abrasive could promote chemical reaction. Yeon et al. [13] studied the chemical reaction simulation of the interface between hydroxylated amorphous silicon and silica, and showed that atom transfer occurred at the interface during the sliding process, which demonstrated that water played an important role at atomic scale. Fogarty J C et al. [19] first demonstrated the feasibility of using ReaxFF to describe the direct interaction between amorphous silica and water. Unlike other classical MD methods and ab initio, ReaxFF generates data about the chemical composition of the system. After the reaction of pure water and silica, amorphous silica surface is covered by SieOH group. However, they did not further study the hydroxylation reaction and the factors that affect the degree of hydroxylation on the surface. Yue et al. [20] studied the sliding process of amorphous silica in phosphoric acid solution and pure
As one of important optical materials, quartz glass has been widely applied in the fields of optical precision instruments, aerospace and radar with the development of optical technology, thus the requirements on the machining qualities rise corresponding. Chemical mechanical polishing (CMP) can achieve global flattening and can be used to process quartz glass [1]. CMP process involves both mechanical actions and chemical interactions (chemical reaction between polishing slurry and workpiece) [2]. At present, it is considered that the removal mechanism during the CMP process mainly include adhesive wear, abrasive wear, impact wear and corrosion wear [3]. The mechanical action of abrasive between workpiece and polishing pad leads to the removal of workpiece, the reaction between workpiece and polishing slurry is a dynamic process which determines the removal rate, surface smoothness and surface defects [4]. The chemical reaction between workpiece and polishing slurry can improve the machinability of workpiece [5], which is of positive significance for the machining process of the brittle-hard materials, such as quartz glass. Many scholars have studied the mechanism of material removal using high-precision microscopic equipment. Katsuki et al. [6,7] revealed the atomic removal mechanism while Si tip sliding on the Si and SiO2 surface in aqueous KOH, demonstrating that the occurrence of SieOeSi during the sliding process caused the Si atoms to be removed.
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Corresponding author.
https://doi.org/10.1016/j.apsusc.2019.144610 Received 11 September 2019; Received in revised form 30 October 2019; Accepted 4 November 2019 Available online 23 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiaoguang Guo, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144610
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3. Results and discussion
water under different pressure using ReaxFF MD. Water molecules can help avoid direct adhesion of amorphous silica surface in pure water under low pressure. Phosphoric acid molecules produce oligomers at low pressure, which can increase surface wear. But they did not delve into the effects of pressure and concentration on removal mechanisms. So far, scholars demonstrated the feasibility of using ReaxFF to describe the direct interaction between amorphous silica and water and studied the effects of water and phosphoric acid on material removal, but no scholars studied the removal mechanism of quartz glass in chemical mechanical polishing in detail, or elucidated the effects of pressure and slurry on the physical-chemical interactions of abrasive and substrate. Pressure and slurry play a vital role in the CMP process, so it is particularly important to elaborate the mechanism of action from the microscopic perspective. Many scholars have studied the CMP process of quartz glass in aqueous environment from the experimental perspective [21–23], but there is little research on the CMP mechanism of quartz glass from the microscopic perspective. In experiments, we can only realize CMP process from macro scale, but we cannot explain the mechanism of removal process in the CMP process, nor can we realize chemical reaction and mechanical effect in detail. Thus it is important for us to shed light into the removal mechanism of quartz glass and effects of polishing parameters on removal mechanism during the CMP process. TBQCMD and ReaxFF MD are widely applied to describe chemical reaction from atomic perspective. Compared with TBQCMD, ReaxFF MD could reduce calculation cost and accelerate simulation speed. In this paper, ReaxFF MD was used to simulate the CMP process of quartz glass in aqueous environment.
3.1. Surface chemical state The pristine quartz glass surface is shown in Fig. 2. After reaction with H2O for 120 ps, the chemical state of quartz glass surface has completely changed. As shown in Fig. 2(a), H2O have two adsorption forms on the quartz glass surface, namely molecular adsorption and dissociative adsorption respectively. The main form of molecular adsorption is that H2O adsorbs on the quartz glass surface. The process of dissociative adsorption is mainly that H2O decomposes into H and OH, and then H and OH combine with the O atoms or undercoordinated Si atoms on the quartz glass surface to form SieOH group bonds, respectively. In order to clarify the mechanism of dissociation and reconnection of H2O, Fig. 2(b) tracked a H2O (marked H1, H2, O1). At 8 ps, the H2O move to quartz glass surface, it can be found that the O2 atom and Si1 atom are in undercoordinated state. Under the action of O2 and Si1, the H2O decomposes into H and OH and combine with O2 and Si1, forming SieOH bond at 11 ps. The reaction process can be described as follows:
(≡Si)+ + (Si − O)− + H2 O ⇌ 2(≡Si − OH )
(1)
The break of SieO bonds and the formation of SieOH bonds play a dual role during the CMP process of quartz glass. Yue et al. [20] calculated the bond energy by ReaxFF and DFT methods and the results indicated that H proton adsorbed on O from SieO bond could reduce the energy needed to break the SieO bonds. El-Sayed et al. [27] used first Principle to prove that H proton could promote the break of SieOeSi bonds. The bond energy of SieO is larger than that of SieOH, so more energy is needed to break the SieO bond in the CMP process. The degree of surface hydroxylation of quartz glass determines the effect of surface softening. To investigate the degree of the dissociation and adsorption of H2O on the quartz glass surface, the number of H2O, SieOH (OH from H2O), SiOeH(H from H2O) and the total number of SieOH bonds were counted. As shown in Fig. 3, at the beginning of simulation, the dissociation rate of H2O and formation rate of SieOH bonds are very fast because of many undercoordinated atoms on the quartz glass surface. Within 20 ps, the number of H2O decreases rapidly, and the number of SieOH bonds increases accordingly. After 20 ps, the reaction between quartz glass and H2O has basically completed and the number of H2O and new bonds are going to stable. In the whole process of reaction, 220 H2O dissociates and 150SieOH bonds are formed, of which about 100 are SieOH (OH from H2O), SiOeH(H from H2O). Aqueous H2O2 affect the chemical state of the quartz glass surface during the CMP process, such as the hydroxyl ratio of surface so as to affect the removal process. To compare the hydroxylation degree of pure H2O and 15% aqueous H2O2 on the quartz glass surface, the reaction between 15% aqueous H2O2 and the quartz glass was simulated, and the results compared with pure H2O are shown in Fig. 4. It can be seen from Fig. 4(a), since there are many free H+ in aqueous H2O2, O atoms on the quartz glass surface combine with free H+ rapidly to form the SiOeH bonds at the beginning of reaction. The number of SiOeH bonds tended to stable over reaction proceeding. Fig. 4(b) shows the number of SieOH bonds formed by combining with OH in pure H2O. The SieOH bonds formed in pure H2O are slightly more than 15% aqueous H2O2. After fully reaction with H2O and aqueous H2O2, about 160 OH group were formed in aqueous H2O2 while about 150 OH group were formed in pure H2O. The reaction rate in aqueous H2O2 is dramatically faster than in pure H2O.
2. Simulation methods In this work, molecular dynamics (MD) simulations using reactive force field (ReaxFF) were utilized to study the CMP process of quartz glass. The removal process as well as the effects of pressure and H2O2 were described in atomic scale. Unlike traditional force fields, ReaxFF utilizes bond order to describe the interactions among atoms, allowing bonds to form and break [17]. To calculate the contribution of each part of the energy, all of bond orders are calculated and modified at each timestep [16]. ReaxFF method has been applied in some CMP simulations [11,12]. In this simulation, the system contains Si, O and H, it is significant to select a proper force field. This force field was developed by J.C Fogarty et al. [19], which contains elements of Si, H and O and trains to model the interaction of water at the SiO2 surface with specific emphasis on proton-transfer reactions. In order to simulate the CMP process of quartz glass in aqueous environment, a model consisting three parts was established: quartz glass substrate (amorphous SiO2), H2O molecule and abrasive particle (SiO2), as shown in Fig. 1. The amorphous silica system was constructed by placing SiO2 molecules randomly in the box, resulting in a silica system with an initial density of 2.2 g/cm3 [19]. Simulation of an amorphous silica slab with the correct structural properties requires very high temperature annealing [24]. The system was annealed twice from 4000 to 300 K. After the amorphous silica model is obtained, the final model is obtained by combining with H2O molecules. The model is constructed in lammps [25,26]. In simulations, three steps were carried out to simulate the CMP process of quartz glass: (1) Reaction between amorphous SiO2 substrate and aqueous for 120 ps. (2) Vertical movement of movable rigid layer of abrasive particle (SiO2) towards the substrate of quartz glass until the target pressure is reached. (3) The abrasive sliding along X-axis positive direction at the speed of 50 m/s. The specific simulation parameters are shown in Table 1.
3.2. Formation mechanism of interface bridge bonds In the CMP process of quartz glass, the formation of interface SieOeSi bonds is very important for the removal of surface materials. The interface bridge bonds play the role of transferring mechanical 2
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Fig. 1. Schematic of the quartz glass in aqueous environment during the CMP process.
from the Si4 atom under the action of abrasive and interface Si1eO2eSi4 bond forms through the dehydroxylation reaction. The existence of H proton weakens the bond energy of SieO bonds. In our simulations, we found that the H proton also promote the formation of interface bridge bonds. As shown in Fig. 6(a), Si6 atom bonds with Si8 atom through the connection of O7 atom in abrasive, and the Si11 atom on the quartz glass surface is hydroxylated by O10H9. With the movement of abrasive, the H9 adsorbs on O7 atom, which promotes the breaking of Si6eO7eSi8 chemical bond and the formation of Si6eO10eSi11 bridge bond, as shown in Fig. 6(b). The movement of abrasive has a positive role on the formation of interface bridge bonds and benefits dehydrogenation and dehydroxylation reactions and makes it easier for abrasive and activated atoms in quartz glass to get closer to form interface bridge bonds.
Table 1 The specific parameters of the CMP simulation. Size of the model Number of H2O Ensemble Temperature (K) Timestep (fs) Sliding speed (m/s) Pressure (GPa)
70 Å×70 Å×90 Å 840 NVT 300 0.05 50 2, 4, 6
forces during the sliding process. Through MD simulations, the formation process of interface bridge bonds between quartz glass and abrasive is elucidated. Fig. 5 shows the formation process of one bridge bond, and the atoms are partially colored to see clearly [28]. As shown in Fig. 5(a), in the initial state, the Si1 atom bonded with O2H3 to form the surface hydroxyl structure. With the movement of abrasive, when the Si1 atom gradually approaches the Si4 atom on the quartz glass surface, the dehydrogenation occurs on Si1eO2H3 structure and the H3 re-bones with O5 atom. As shown in Fig. 5(c), the O5H3 is separated
3.3. Surface atoms removal process To further explain the removal mechanism of quartz glass during the CMP process, some atoms removed were tracked. ReaxFF judges the
Fig. 2. Interface state of quartz glass after reaction. 3
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3.4. Effect of the pressure CMP experiments and microscopic atomic force microscope (AFM) experiments showed that the removal amount of quartz glass increases with the increment of pressure. In order to study the effect of pressure on material removal mechanism in the system, the surfaces of abrasive after sliding process are obtained by changing the interface pressure (2.0 GPa, 4.0 GPa, 6.0 GPa). By analyzing the products on the surface, the removal amount and removal form of quartz glass surface atoms are obtained. As shown in Fig. 8(a), under the pressure of 2.0 GPa, only 4 Si atoms and 11 O atoms are removed. The number of atoms removed increases rapidly with the increment of pressure, there are 9 Si atoms and 22 O atoms are removed under the pressure of 4.0 GPa. When the pressure reaches 6.0 GPa, the number of Si atom removed and O atom is 29 and 62, respectively. From previous analysis, the formation of bridge bonds between quartz glass surface and abrasive is of vital importance. The bridge bonds between quartz glass surface and abrasive are broken under the stretching of abrasive, leading to the removal of surface atoms. Therefore, the increment in the number of atoms removed is related to the number of interface bridge bonds. To verify this inference, the number of interface bridge bonds over sliding time under different pressures are calculated, as shown in Fig. 8(b). The figure indicates that under the pressure of 2.0 GPa, only a few bridge bonds are formed during the sliding process. And with the increment of pressure, the number of bridge bonds increases rapidly. This is mainly because that the water layer at the interface prevent the contact between abrasive and quartz glass at lower pressure. However, the water layer cannot completely prevent the contact between them at higher pressure, resulting in the increment of bridge bonds. The fluctuation of the curves demonstrates that the bridge bonds are constantly formed and broken during the sliding process. The interface pressure not only affects the removal amount and the formation of bridge bonds, but also affects the removal forms. Fig. 9 shows chemical state of the abrasive surface after sliding process under different pressures. In order to observe the existing state of removed atoms, the symmetrical transformation of the abrasive was carried out. As mentioned earlier, only a few bridge bonds between abrasive and quartz glass are formed under the pressure of 2 GPa, leading to only 4 Si atoms are removed, as shown in Fig. 9(a), the removal form of Si atoms is single atom removal. When the interface pressure increases from 2 GPa to 4 GPa, chain removal form can be seen from Fig. 9(b). As shown in Fig. 9(b), three adjacent Si atoms are removed (Si2eOeSi3O7H3), in this chain removal, Si atoms on the quartz glass surface form two bridge bonds with abrasive. Under the action of the two bridge bonds, three adjacent Si atoms and O atoms are removed.
Fig. 3. The number of H2O and SieOH bonds at amorphous silica-water interface.
breakage or formation of chemical bonds among atoms based on bond order, so the information of chemical bonds can be obtained by calculating the changes of bond order among atoms. Fig. 7 shows the removal process of quartz glass surface atoms and the changes of bond order among atoms. As shown in Fig. 7(a) and (b), the Si atoms on the surfaces of abrasive and quartz glass are basically in full coordination state and there are a lot of SieOH bonds on the quartz glass surface, and the Si1 atom on the quartz glass surface is hydroxylated by O2H from H2O, the O6 atom on the abrasive surface bonded with a free H11 proton to form SieOH bonds. Fig. 7(c) represents the changes of bond order among atoms during the sliding process of quartz glass. As shown in Fig. 7(c), the bond order of SieO is about 0.94Kcal/mole. In Fig. 7(c1), the bond order of Si1eO2 decreases rapidly at 12 ps, indicating that the Si1eO2 bond is broken at this time, then the bond order of Si1eO6 increases after the break of Si1eO2 bond, as shown in Fig. 7(c2), indicating that the Si1eO2eSi12 bond has formed. The chemical bonds between Si1 atom and quartz glass substrate atoms are broken by the mechanical force transmitted through the interface bridge bonds. The chemical bonds are generated and broken continuously, and under the synergistic action of several bridge bonds, the Si1 atom is finally removed, as shown in Fig. 7(c3) and (c4).
Fig. 4. Number of SiOeH and SieOH bonds on the quartz glass surface. 4
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Fig. 5. The formation process of interface bridge bond through dehydrogenation and dehydroxylation reactions: (a) initial state (b) dehydrogenation of Si1eO2H3 (c) formation of Si1eO2eSi4 bridge bond.
number of chemical bonds increases over the increment of contact area. When the ratio of SieOH group on the surface is 30%, there are many OH groups on the quartz glass and abrasive surface. The interface bridge bonds are formed over dehydrogenation and dehydroxylation, so when the interface contact pressure and area are small, the mechanical force introduced by the abrasive is not enough to make the OH group on quartz glass surface destroyed rapidly, thus the formation speed of bridge bonds is lower than that of 0%. With the increment of contact pressure and area, more OH group are destroyed by mechanical force, therefore, the number of chemical bonds increase rapidly over the proceeding of sliding, and the number of bridge bonds is basically same as the situation of 0%. When the ratio of SieOH group is 50%, excessive OH group inhibit the contact of abrasive and quartz glass surface and larger mechanical force is needed to destroy OH group. Therefore, the number of bridge bonds is less than the two situations under the same pressure. The degree of hydroxylation on the quartz glass surface not only affects the number of atoms removed, but also affects the removed forms of surface atoms. Fig. 12 is the distributions of atoms removed on the initial surface. When the ratio of SieOH group is 0%, the surface atoms are mainly removed in the form of single Si atom along with the surrounding bonded O atoms, and the atoms removed are widely distributed. When the ratio of SieOH group on the quartz glass surface is 30%, it is obvious that the atoms removed are concentrated in one region. The removal form of surface atoms is chain removal at this situation. This is because that the OH group in this region are destroyed by the mechanical force, many SieOeSi bonds are formed in this area over dehydrogenation and dehydroxylation. Under the synergistic effect of bridge bonds and mechanical forces, the chemical bonds between the atoms removed and the quartz glass substrate are broken. When the ratio is 50%, the atoms removed decrease obviously due to the decrease of bridge bonds.
When the pressure is 6 GPa, the number of removed atoms is dramatically increased, as shown in Fig. 9(c), most atoms are removed in the form of chain.
3.5. Effect of aqueous H2O2 Aqueous H2O2 will affect the surface chemical state of quartz glass in the CMP process, such as the degree of surface hydroxylation so as to affect the removal of surface atoms. Many CMP experiments manifested that the concentration of aqueous H2O2 has a great influence on the removal rate and surface roughness. Yoomin Ahn et al. [29] carried out CMP experiments on aluminum with silica slurry, the results indicated that as the concentration of aqueous H2O2, the removal rate increases firstly and then decreases, and the roughness decreases in this process. Previous analysis has proved that aqueous H2O2 influences the hydroxylation degree of quartz glass surface. To reveal the relationship between the hydroxylation degree and the removal mechanism of surface atoms from the atomic scale, the model as shown in Fig. 10 was established. Different hydroxylation ratios on the quartz glass surface were obtained by adding different amounts of H atoms and OH group to the quartz glass surface [28]: (1) fresh quartz glass surface without hydroxyl group (ratio of SieOH group on the surface is 0%); (2) adding 20 H atoms and 14 OH groups (30%); (3) adding 30 H atoms and 25 OH groups (50%). The surface of abrasive is fully hydroxylated and the pressure on the movable rigid layer is 2GPa. Other settings are same as before. Fig. 11(a) depicts the number of Si atoms and O atoms removed under different hydroxylation degree. The removal amount of surface atoms increases firstly and then decreases over the increment of the hydroxylation degree. The number of chemical bonds formed during the sliding process under different hydroxylation degrees are shown in Fig. 11(b). There are many under-coordinated atoms when the pristine quartz glass surface without OH group, so interface bridge bonds formed rapidly when the abrasive is close to the surface, and the
Fig. 6. Promotion of H proton on the formation of interface bridge bonds: (a) initial state (b) promotion of H proton. 5
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Fig. 7. The formation of bridge bond and removal path of surface atoms: (a) position of the tracked atoms during the sliding process (b) initial state of the tracked atoms (c) changes of bond order during the sliding process.
(a) number of atoms removed
(b) number of bridge bonds
Fig. 8. The effect of pressure on the number of atoms removed and formation of bridge bonds.
4. Conclusion
affecting the degree of hydroxylation on the quartz glass surface. With the increment of hydroxylation degree, the removal amount of surface atoms increases firstly and then decreases, which is agree with the experiment results. In the case of the hydroxylation degree is 0% or 50%, the removal form is single atom removal, but when the hydroxylation is 30%, the removal form is chain removal. In addition, the role of pressure was investigated. Under the action of interface pressure, SieOeSi bonds are formed between quartz glass surface and abrasive. There two ways to form interface bridge bonds, namely
In this paper, the removal mechanism of quartz glass in the CMP process has been elucidated utilizing ReaxFF MD. After fully reacted with H2O, quartz glass surface will be hydroxylated. There are two adsorption forms of water on the quartz glass surface, namely dissociative adsorption and molecular adsorption. Compared with pure H2O, the hydroxylation degree is higher in aqueous H2O2 due to the presence of H2O2. Aqueous H2O2 affects the removal process by 6
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(a) 2GPa
(b) 4GPa
(c) 6GPa Fig. 9. Chemical state of the abrasive surface after sliding process under different pressures.
Fig. 10. Model for the study on the effect of aqueous H2O2.
interests or personal relationships that could have appeared to influence the work reported in this paper.
dehydrogenation and dehydroxylation. With the increment of interface pressure, the number of atoms removed increases because of more bridge bonds forming under larger pressure. The removal form of surface atoms changes from single atom removal to chain removal over the increment of pressure. Aqueous H2O2 and pressure have great influences on the removal amount and removal forms. Therefore, the atoms on the quartz glass surface are removed under the synergistic action of mechanical forces and chemical reactions. This work helps us reveal the removal mechanism in the CMP process of quartz glass from an atomic perspective.
Acknowledgments The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (General Program) (NO. 51575083), Science Fund for Creative Research Groups (NO. 51621064) and the National Natural Science of China (NO. 51505063). ReaxFF MD simulations were carried out at LvLiang Cloud Computing Center of China, and the calculations were performed on TianHe-2.
Declaration of Competing Interest The authors declare that they have no known competing financial 7
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Fig. 11. The effect of hydroxylation degree on the number of atoms removed.
(a) 0%
(b) 30%
(c) 50 %
Fig. 12. Distribution of atoms removed on the initial surface (red parts represent the atoms removed).
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Full Length Article
Effects of pressure and slurry on removal mechanism during the chemical mechanical polishing of quartz glass using ReaxFF MD ⁎
Xiaoguang Guo, Song Yuan , Junxin Huang, Chong Chen, Renke Kang, Zhuji Jin, Dongming Guo Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Quartz glass CMP ReaxFF MD Removal mechanism Aqueous H2O2 Pressure
Reactive force field molecular dynamics (ReaxFF MD) simulation was employed to study the interfacial interaction during the Chemical mechanical polishing (CMP) process of quartz glass. Results indicated that dissociation and adsorption of H2O occurred on the quartz glass surface in pure H2O, and the dissociation of H2O could hydroxylate quartz glass surface to form SieOH bonds. In addition, it was found that the surface hydroxylation degree is higher in aqueous H2O2. The formation of SieOeSi bonds plays a key role in the CMP process. Dehydrogenation and dehydroxylation are two ways to form interface bridge bonds. The interface bridge bonds between abrasive and substrate could transmit mechanical force with the movement of abrasive. Interface pressure affects the number and forms of atoms removed. With the increment of pressure, the removal form gradually changes from single removal mode to chain removal mode. The effect of aqueous H2O2 on the removal process was also studied by adding different amount of OH group on the quartz glass surface. This work helps us reveal the removal mechanism in the CMP process of quartz glass from an atomic perspective.
1. Introduction
Shen N et al. [8] reported the mechanics of polishing particles near the elastic-plastic load boundary of optical glass by AFM. They also proposed that the surface densification of different optical glasses was different during the CMP process. Molecular dynamics (MD) simulation could explain the mechanism from the atomic perspective and reveal the details of particle interaction in the CMP process. Now, simulation technologies such as tight-binding quantum chemical molecular dynamics (TBQC-MD) [1,9,10] as well as reactive force field (ReaxFF) [11–17] have been applied in MD simulation successfully. Kubo et al. [10,18] performed the SiO2 CMP process with CeO2 abrasive by TBQCMD, indicating that mechanical force induced by CeO2 abrasive could promote chemical reaction. Yeon et al. [13] studied the chemical reaction simulation of the interface between hydroxylated amorphous silicon and silica, and showed that atom transfer occurred at the interface during the sliding process, which demonstrated that water played an important role at atomic scale. Fogarty J C et al. [19] first demonstrated the feasibility of using ReaxFF to describe the direct interaction between amorphous silica and water. Unlike other classical MD methods and ab initio, ReaxFF generates data about the chemical composition of the system. After the reaction of pure water and silica, amorphous silica surface is covered by SieOH group. However, they did not further study the hydroxylation reaction and the factors that affect the degree of hydroxylation on the surface. Yue et al. [20] studied the sliding process of amorphous silica in phosphoric acid solution and pure
As one of important optical materials, quartz glass has been widely applied in the fields of optical precision instruments, aerospace and radar with the development of optical technology, thus the requirements on the machining qualities rise corresponding. Chemical mechanical polishing (CMP) can achieve global flattening and can be used to process quartz glass [1]. CMP process involves both mechanical actions and chemical interactions (chemical reaction between polishing slurry and workpiece) [2]. At present, it is considered that the removal mechanism during the CMP process mainly include adhesive wear, abrasive wear, impact wear and corrosion wear [3]. The mechanical action of abrasive between workpiece and polishing pad leads to the removal of workpiece, the reaction between workpiece and polishing slurry is a dynamic process which determines the removal rate, surface smoothness and surface defects [4]. The chemical reaction between workpiece and polishing slurry can improve the machinability of workpiece [5], which is of positive significance for the machining process of the brittle-hard materials, such as quartz glass. Many scholars have studied the mechanism of material removal using high-precision microscopic equipment. Katsuki et al. [6,7] revealed the atomic removal mechanism while Si tip sliding on the Si and SiO2 surface in aqueous KOH, demonstrating that the occurrence of SieOeSi during the sliding process caused the Si atoms to be removed.
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Corresponding author.
https://doi.org/10.1016/j.apsusc.2019.144610 Received 11 September 2019; Received in revised form 30 October 2019; Accepted 4 November 2019 Available online 23 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiaoguang Guo, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144610
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3. Results and discussion
water under different pressure using ReaxFF MD. Water molecules can help avoid direct adhesion of amorphous silica surface in pure water under low pressure. Phosphoric acid molecules produce oligomers at low pressure, which can increase surface wear. But they did not delve into the effects of pressure and concentration on removal mechanisms. So far, scholars demonstrated the feasibility of using ReaxFF to describe the direct interaction between amorphous silica and water and studied the effects of water and phosphoric acid on material removal, but no scholars studied the removal mechanism of quartz glass in chemical mechanical polishing in detail, or elucidated the effects of pressure and slurry on the physical-chemical interactions of abrasive and substrate. Pressure and slurry play a vital role in the CMP process, so it is particularly important to elaborate the mechanism of action from the microscopic perspective. Many scholars have studied the CMP process of quartz glass in aqueous environment from the experimental perspective [21–23], but there is little research on the CMP mechanism of quartz glass from the microscopic perspective. In experiments, we can only realize CMP process from macro scale, but we cannot explain the mechanism of removal process in the CMP process, nor can we realize chemical reaction and mechanical effect in detail. Thus it is important for us to shed light into the removal mechanism of quartz glass and effects of polishing parameters on removal mechanism during the CMP process. TBQCMD and ReaxFF MD are widely applied to describe chemical reaction from atomic perspective. Compared with TBQCMD, ReaxFF MD could reduce calculation cost and accelerate simulation speed. In this paper, ReaxFF MD was used to simulate the CMP process of quartz glass in aqueous environment.
3.1. Surface chemical state The pristine quartz glass surface is shown in Fig. 2. After reaction with H2O for 120 ps, the chemical state of quartz glass surface has completely changed. As shown in Fig. 2(a), H2O have two adsorption forms on the quartz glass surface, namely molecular adsorption and dissociative adsorption respectively. The main form of molecular adsorption is that H2O adsorbs on the quartz glass surface. The process of dissociative adsorption is mainly that H2O decomposes into H and OH, and then H and OH combine with the O atoms or undercoordinated Si atoms on the quartz glass surface to form SieOH group bonds, respectively. In order to clarify the mechanism of dissociation and reconnection of H2O, Fig. 2(b) tracked a H2O (marked H1, H2, O1). At 8 ps, the H2O move to quartz glass surface, it can be found that the O2 atom and Si1 atom are in undercoordinated state. Under the action of O2 and Si1, the H2O decomposes into H and OH and combine with O2 and Si1, forming SieOH bond at 11 ps. The reaction process can be described as follows:
(≡Si)+ + (Si − O)− + H2 O ⇌ 2(≡Si − OH )
(1)
The break of SieO bonds and the formation of SieOH bonds play a dual role during the CMP process of quartz glass. Yue et al. [20] calculated the bond energy by ReaxFF and DFT methods and the results indicated that H proton adsorbed on O from SieO bond could reduce the energy needed to break the SieO bonds. El-Sayed et al. [27] used first Principle to prove that H proton could promote the break of SieOeSi bonds. The bond energy of SieO is larger than that of SieOH, so more energy is needed to break the SieO bond in the CMP process. The degree of surface hydroxylation of quartz glass determines the effect of surface softening. To investigate the degree of the dissociation and adsorption of H2O on the quartz glass surface, the number of H2O, SieOH (OH from H2O), SiOeH(H from H2O) and the total number of SieOH bonds were counted. As shown in Fig. 3, at the beginning of simulation, the dissociation rate of H2O and formation rate of SieOH bonds are very fast because of many undercoordinated atoms on the quartz glass surface. Within 20 ps, the number of H2O decreases rapidly, and the number of SieOH bonds increases accordingly. After 20 ps, the reaction between quartz glass and H2O has basically completed and the number of H2O and new bonds are going to stable. In the whole process of reaction, 220 H2O dissociates and 150SieOH bonds are formed, of which about 100 are SieOH (OH from H2O), SiOeH(H from H2O). Aqueous H2O2 affect the chemical state of the quartz glass surface during the CMP process, such as the hydroxyl ratio of surface so as to affect the removal process. To compare the hydroxylation degree of pure H2O and 15% aqueous H2O2 on the quartz glass surface, the reaction between 15% aqueous H2O2 and the quartz glass was simulated, and the results compared with pure H2O are shown in Fig. 4. It can be seen from Fig. 4(a), since there are many free H+ in aqueous H2O2, O atoms on the quartz glass surface combine with free H+ rapidly to form the SiOeH bonds at the beginning of reaction. The number of SiOeH bonds tended to stable over reaction proceeding. Fig. 4(b) shows the number of SieOH bonds formed by combining with OH in pure H2O. The SieOH bonds formed in pure H2O are slightly more than 15% aqueous H2O2. After fully reaction with H2O and aqueous H2O2, about 160 OH group were formed in aqueous H2O2 while about 150 OH group were formed in pure H2O. The reaction rate in aqueous H2O2 is dramatically faster than in pure H2O.
2. Simulation methods In this work, molecular dynamics (MD) simulations using reactive force field (ReaxFF) were utilized to study the CMP process of quartz glass. The removal process as well as the effects of pressure and H2O2 were described in atomic scale. Unlike traditional force fields, ReaxFF utilizes bond order to describe the interactions among atoms, allowing bonds to form and break [17]. To calculate the contribution of each part of the energy, all of bond orders are calculated and modified at each timestep [16]. ReaxFF method has been applied in some CMP simulations [11,12]. In this simulation, the system contains Si, O and H, it is significant to select a proper force field. This force field was developed by J.C Fogarty et al. [19], which contains elements of Si, H and O and trains to model the interaction of water at the SiO2 surface with specific emphasis on proton-transfer reactions. In order to simulate the CMP process of quartz glass in aqueous environment, a model consisting three parts was established: quartz glass substrate (amorphous SiO2), H2O molecule and abrasive particle (SiO2), as shown in Fig. 1. The amorphous silica system was constructed by placing SiO2 molecules randomly in the box, resulting in a silica system with an initial density of 2.2 g/cm3 [19]. Simulation of an amorphous silica slab with the correct structural properties requires very high temperature annealing [24]. The system was annealed twice from 4000 to 300 K. After the amorphous silica model is obtained, the final model is obtained by combining with H2O molecules. The model is constructed in lammps [25,26]. In simulations, three steps were carried out to simulate the CMP process of quartz glass: (1) Reaction between amorphous SiO2 substrate and aqueous for 120 ps. (2) Vertical movement of movable rigid layer of abrasive particle (SiO2) towards the substrate of quartz glass until the target pressure is reached. (3) The abrasive sliding along X-axis positive direction at the speed of 50 m/s. The specific simulation parameters are shown in Table 1.
3.2. Formation mechanism of interface bridge bonds In the CMP process of quartz glass, the formation of interface SieOeSi bonds is very important for the removal of surface materials. The interface bridge bonds play the role of transferring mechanical 2
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Fig. 1. Schematic of the quartz glass in aqueous environment during the CMP process.
from the Si4 atom under the action of abrasive and interface Si1eO2eSi4 bond forms through the dehydroxylation reaction. The existence of H proton weakens the bond energy of SieO bonds. In our simulations, we found that the H proton also promote the formation of interface bridge bonds. As shown in Fig. 6(a), Si6 atom bonds with Si8 atom through the connection of O7 atom in abrasive, and the Si11 atom on the quartz glass surface is hydroxylated by O10H9. With the movement of abrasive, the H9 adsorbs on O7 atom, which promotes the breaking of Si6eO7eSi8 chemical bond and the formation of Si6eO10eSi11 bridge bond, as shown in Fig. 6(b). The movement of abrasive has a positive role on the formation of interface bridge bonds and benefits dehydrogenation and dehydroxylation reactions and makes it easier for abrasive and activated atoms in quartz glass to get closer to form interface bridge bonds.
Table 1 The specific parameters of the CMP simulation. Size of the model Number of H2O Ensemble Temperature (K) Timestep (fs) Sliding speed (m/s) Pressure (GPa)
70 Å×70 Å×90 Å 840 NVT 300 0.05 50 2, 4, 6
forces during the sliding process. Through MD simulations, the formation process of interface bridge bonds between quartz glass and abrasive is elucidated. Fig. 5 shows the formation process of one bridge bond, and the atoms are partially colored to see clearly [28]. As shown in Fig. 5(a), in the initial state, the Si1 atom bonded with O2H3 to form the surface hydroxyl structure. With the movement of abrasive, when the Si1 atom gradually approaches the Si4 atom on the quartz glass surface, the dehydrogenation occurs on Si1eO2H3 structure and the H3 re-bones with O5 atom. As shown in Fig. 5(c), the O5H3 is separated
3.3. Surface atoms removal process To further explain the removal mechanism of quartz glass during the CMP process, some atoms removed were tracked. ReaxFF judges the
Fig. 2. Interface state of quartz glass after reaction. 3
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3.4. Effect of the pressure CMP experiments and microscopic atomic force microscope (AFM) experiments showed that the removal amount of quartz glass increases with the increment of pressure. In order to study the effect of pressure on material removal mechanism in the system, the surfaces of abrasive after sliding process are obtained by changing the interface pressure (2.0 GPa, 4.0 GPa, 6.0 GPa). By analyzing the products on the surface, the removal amount and removal form of quartz glass surface atoms are obtained. As shown in Fig. 8(a), under the pressure of 2.0 GPa, only 4 Si atoms and 11 O atoms are removed. The number of atoms removed increases rapidly with the increment of pressure, there are 9 Si atoms and 22 O atoms are removed under the pressure of 4.0 GPa. When the pressure reaches 6.0 GPa, the number of Si atom removed and O atom is 29 and 62, respectively. From previous analysis, the formation of bridge bonds between quartz glass surface and abrasive is of vital importance. The bridge bonds between quartz glass surface and abrasive are broken under the stretching of abrasive, leading to the removal of surface atoms. Therefore, the increment in the number of atoms removed is related to the number of interface bridge bonds. To verify this inference, the number of interface bridge bonds over sliding time under different pressures are calculated, as shown in Fig. 8(b). The figure indicates that under the pressure of 2.0 GPa, only a few bridge bonds are formed during the sliding process. And with the increment of pressure, the number of bridge bonds increases rapidly. This is mainly because that the water layer at the interface prevent the contact between abrasive and quartz glass at lower pressure. However, the water layer cannot completely prevent the contact between them at higher pressure, resulting in the increment of bridge bonds. The fluctuation of the curves demonstrates that the bridge bonds are constantly formed and broken during the sliding process. The interface pressure not only affects the removal amount and the formation of bridge bonds, but also affects the removal forms. Fig. 9 shows chemical state of the abrasive surface after sliding process under different pressures. In order to observe the existing state of removed atoms, the symmetrical transformation of the abrasive was carried out. As mentioned earlier, only a few bridge bonds between abrasive and quartz glass are formed under the pressure of 2 GPa, leading to only 4 Si atoms are removed, as shown in Fig. 9(a), the removal form of Si atoms is single atom removal. When the interface pressure increases from 2 GPa to 4 GPa, chain removal form can be seen from Fig. 9(b). As shown in Fig. 9(b), three adjacent Si atoms are removed (Si2eOeSi3O7H3), in this chain removal, Si atoms on the quartz glass surface form two bridge bonds with abrasive. Under the action of the two bridge bonds, three adjacent Si atoms and O atoms are removed.
Fig. 3. The number of H2O and SieOH bonds at amorphous silica-water interface.
breakage or formation of chemical bonds among atoms based on bond order, so the information of chemical bonds can be obtained by calculating the changes of bond order among atoms. Fig. 7 shows the removal process of quartz glass surface atoms and the changes of bond order among atoms. As shown in Fig. 7(a) and (b), the Si atoms on the surfaces of abrasive and quartz glass are basically in full coordination state and there are a lot of SieOH bonds on the quartz glass surface, and the Si1 atom on the quartz glass surface is hydroxylated by O2H from H2O, the O6 atom on the abrasive surface bonded with a free H11 proton to form SieOH bonds. Fig. 7(c) represents the changes of bond order among atoms during the sliding process of quartz glass. As shown in Fig. 7(c), the bond order of SieO is about 0.94Kcal/mole. In Fig. 7(c1), the bond order of Si1eO2 decreases rapidly at 12 ps, indicating that the Si1eO2 bond is broken at this time, then the bond order of Si1eO6 increases after the break of Si1eO2 bond, as shown in Fig. 7(c2), indicating that the Si1eO2eSi12 bond has formed. The chemical bonds between Si1 atom and quartz glass substrate atoms are broken by the mechanical force transmitted through the interface bridge bonds. The chemical bonds are generated and broken continuously, and under the synergistic action of several bridge bonds, the Si1 atom is finally removed, as shown in Fig. 7(c3) and (c4).
Fig. 4. Number of SiOeH and SieOH bonds on the quartz glass surface. 4
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Fig. 5. The formation process of interface bridge bond through dehydrogenation and dehydroxylation reactions: (a) initial state (b) dehydrogenation of Si1eO2H3 (c) formation of Si1eO2eSi4 bridge bond.
number of chemical bonds increases over the increment of contact area. When the ratio of SieOH group on the surface is 30%, there are many OH groups on the quartz glass and abrasive surface. The interface bridge bonds are formed over dehydrogenation and dehydroxylation, so when the interface contact pressure and area are small, the mechanical force introduced by the abrasive is not enough to make the OH group on quartz glass surface destroyed rapidly, thus the formation speed of bridge bonds is lower than that of 0%. With the increment of contact pressure and area, more OH group are destroyed by mechanical force, therefore, the number of chemical bonds increase rapidly over the proceeding of sliding, and the number of bridge bonds is basically same as the situation of 0%. When the ratio of SieOH group is 50%, excessive OH group inhibit the contact of abrasive and quartz glass surface and larger mechanical force is needed to destroy OH group. Therefore, the number of bridge bonds is less than the two situations under the same pressure. The degree of hydroxylation on the quartz glass surface not only affects the number of atoms removed, but also affects the removed forms of surface atoms. Fig. 12 is the distributions of atoms removed on the initial surface. When the ratio of SieOH group is 0%, the surface atoms are mainly removed in the form of single Si atom along with the surrounding bonded O atoms, and the atoms removed are widely distributed. When the ratio of SieOH group on the quartz glass surface is 30%, it is obvious that the atoms removed are concentrated in one region. The removal form of surface atoms is chain removal at this situation. This is because that the OH group in this region are destroyed by the mechanical force, many SieOeSi bonds are formed in this area over dehydrogenation and dehydroxylation. Under the synergistic effect of bridge bonds and mechanical forces, the chemical bonds between the atoms removed and the quartz glass substrate are broken. When the ratio is 50%, the atoms removed decrease obviously due to the decrease of bridge bonds.
When the pressure is 6 GPa, the number of removed atoms is dramatically increased, as shown in Fig. 9(c), most atoms are removed in the form of chain.
3.5. Effect of aqueous H2O2 Aqueous H2O2 will affect the surface chemical state of quartz glass in the CMP process, such as the degree of surface hydroxylation so as to affect the removal of surface atoms. Many CMP experiments manifested that the concentration of aqueous H2O2 has a great influence on the removal rate and surface roughness. Yoomin Ahn et al. [29] carried out CMP experiments on aluminum with silica slurry, the results indicated that as the concentration of aqueous H2O2, the removal rate increases firstly and then decreases, and the roughness decreases in this process. Previous analysis has proved that aqueous H2O2 influences the hydroxylation degree of quartz glass surface. To reveal the relationship between the hydroxylation degree and the removal mechanism of surface atoms from the atomic scale, the model as shown in Fig. 10 was established. Different hydroxylation ratios on the quartz glass surface were obtained by adding different amounts of H atoms and OH group to the quartz glass surface [28]: (1) fresh quartz glass surface without hydroxyl group (ratio of SieOH group on the surface is 0%); (2) adding 20 H atoms and 14 OH groups (30%); (3) adding 30 H atoms and 25 OH groups (50%). The surface of abrasive is fully hydroxylated and the pressure on the movable rigid layer is 2GPa. Other settings are same as before. Fig. 11(a) depicts the number of Si atoms and O atoms removed under different hydroxylation degree. The removal amount of surface atoms increases firstly and then decreases over the increment of the hydroxylation degree. The number of chemical bonds formed during the sliding process under different hydroxylation degrees are shown in Fig. 11(b). There are many under-coordinated atoms when the pristine quartz glass surface without OH group, so interface bridge bonds formed rapidly when the abrasive is close to the surface, and the
Fig. 6. Promotion of H proton on the formation of interface bridge bonds: (a) initial state (b) promotion of H proton. 5
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Fig. 7. The formation of bridge bond and removal path of surface atoms: (a) position of the tracked atoms during the sliding process (b) initial state of the tracked atoms (c) changes of bond order during the sliding process.
(a) number of atoms removed
(b) number of bridge bonds
Fig. 8. The effect of pressure on the number of atoms removed and formation of bridge bonds.
4. Conclusion
affecting the degree of hydroxylation on the quartz glass surface. With the increment of hydroxylation degree, the removal amount of surface atoms increases firstly and then decreases, which is agree with the experiment results. In the case of the hydroxylation degree is 0% or 50%, the removal form is single atom removal, but when the hydroxylation is 30%, the removal form is chain removal. In addition, the role of pressure was investigated. Under the action of interface pressure, SieOeSi bonds are formed between quartz glass surface and abrasive. There two ways to form interface bridge bonds, namely
In this paper, the removal mechanism of quartz glass in the CMP process has been elucidated utilizing ReaxFF MD. After fully reacted with H2O, quartz glass surface will be hydroxylated. There are two adsorption forms of water on the quartz glass surface, namely dissociative adsorption and molecular adsorption. Compared with pure H2O, the hydroxylation degree is higher in aqueous H2O2 due to the presence of H2O2. Aqueous H2O2 affects the removal process by 6
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(a) 2GPa
(b) 4GPa
(c) 6GPa Fig. 9. Chemical state of the abrasive surface after sliding process under different pressures.
Fig. 10. Model for the study on the effect of aqueous H2O2.
interests or personal relationships that could have appeared to influence the work reported in this paper.
dehydrogenation and dehydroxylation. With the increment of interface pressure, the number of atoms removed increases because of more bridge bonds forming under larger pressure. The removal form of surface atoms changes from single atom removal to chain removal over the increment of pressure. Aqueous H2O2 and pressure have great influences on the removal amount and removal forms. Therefore, the atoms on the quartz glass surface are removed under the synergistic action of mechanical forces and chemical reactions. This work helps us reveal the removal mechanism in the CMP process of quartz glass from an atomic perspective.
Acknowledgments The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (General Program) (NO. 51575083), Science Fund for Creative Research Groups (NO. 51621064) and the National Natural Science of China (NO. 51505063). ReaxFF MD simulations were carried out at LvLiang Cloud Computing Center of China, and the calculations were performed on TianHe-2.
Declaration of Competing Interest The authors declare that they have no known competing financial 7
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Fig. 11. The effect of hydroxylation degree on the number of atoms removed.
(a) 0%
(b) 30%
(c) 50 %
Fig. 12. Distribution of atoms removed on the initial surface (red parts represent the atoms removed).
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