Chemical Mechanical Polishing Mechanism Research Articles

Atomic surface induced by novel green chemical mechanical polishing for aspheric thin-walled crucibles with large diameters

Aspheric thin-walled crucibles with large diameters are widely used for photovoltaics and semiconductor industries, while they have hard-brittle and aspheric characteristics. Nevertheless, these crucibles strictly require atomic surfaces and high material removal rate (MRR). This is a challenge for chemical mechanical polishing (CMP). To break this challenge, a novel green CMP was developed for fused silica, containing ceria, lanthana, hydrogen peroxide, diethylene triaminepentaacetic acid pentasodium, carboxymethyl cellulose sodium and guanidine carbonate. After CMP, surface roughness Ra of 0.115 is obtained at a scanning area of 5 × 5 μm2 measured by atomic force microscopy, and MRR is 563 nm/min. In such an atomic surface, the MRR is the highest compared with those published previously. Transmission electron microscopy confirms that the thickness of damaged layer after CMP is only 2.63 nm. A novel approach of robot floating polishing is proposed for aspheric crucibles with a diameter of 810 mm and a height of 580 mm. Using the developed CMP, surface roughness Sa of 0.178 nm on inner surface of an aspheric thin-walled crucible is achieved under a measurement area of 50 × 50 μm2. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance reveal that the content of Ce3+ increased from 28.54 to 43.76 %. Through Raman, XPS and Fourier transform infrared spectroscopy, CMP mechanisms are elucidated. Densified damaged layer on the surface of fused silica was induced by the transformation of surface structure from six-membered to four- or three-membered network. Ce-OH and Si-OH were formed between fused silica and ceria abrasives, forming Ce-O-Si by condensation reaction of dehydration. The developed green CMP and approach provide new pathway to polish and manufacture atomic surfaces for aspheric thin-walled crucibles with large diameters.

Development of core–shell SiO2@A-TiO2 abrasives and novel photocatalytic chemical machinal polishing for atomic surface of fused silica

It is extremely hard to achieve atomic surface on fused silica with a high material removal rate (MRR). Furthermore, in traditional chemical mechanical polishing (CMP), toxic and corrosive slurries are widely employed. To address these challenges, core–shell SiO2@A-TiO2 abrasives were prepared, and novel photocatalytic CMP was developed. The developed CMP slurry consists of sodium carbonate, hydrogen peroxide, sodium carboxymethylcellulose and deionized water. After CMP, atomic surface with surface roughness Sa of 0.181 nm was achieved at a scanning area of 50 × 50 μm2 and a high MRR of 10.727 μm/h. X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared were used to elucidate the photocatalytic CMP mechanisms. The developed SiO2@A-TiO2 abrasives could generate electrons and holes irradiated by simulated solar light, producing free radicals (OH–). As a result, OH– combined with Si atoms on the surface of fused silica, forming Si-OH-Si bonds. This constructed interlinkage of slurry and surface of fused silica via SiO2@A-TiO2 abrasives improved synergistic effect between chemical and mechanical functions. The developed core–shell abrasives and photocatalytic CMP suggest new findings to achieve atomic surface for a hard-brittle solid with a high MRR.

Investigation of CMP Process of Silica Substrate by Ceria Abrasive: Insights from Molecular Dynamics Simulation with Neural Network Potential

Chemical mechanical polishing (CMP) is a process of surface planarization by chemical and mechanical driving forces. CMP is expected to solve various problems, including the surface roughness on semiconductor devices. An aqueous slurry, including abrasive grains and chemical components, is used in the CMP process. Development of slurries with high polishing rate, excellent planarity, and superior processability is not only a key challenge, but also a significant opportunity for the future of our industry. In order to achieve the development, it is necessary to understand the detailed mechanism of CMP, which involves complex phenomena accompanied by interfacial chemical reactions and mechanical friction. Computational scientific approaches, such as density functional theory (DFT) and molecular dynamics (MD) simulations, are powerful tools for understanding complex tribochemical phenomena at the atomic scale. However, these methods require much computation time and present difficulties for large-scale simulations. To overcome this limitation, a computational method called Neural Network Potential (NNP) is a powerful tool because it achieves a good balance between accuracy and computational efficiency by constructing potential energy surfaces based on a neural network-based method. We performed NNP-MD simulations of the ceria/silica interface with the development of CMP slurry including ceria abrasive grains, which is typically used on the surfaces of silicon layered devices. As a result, we succeeded in observing polishing phenomena at the interface at the atomic scale, namely Ce-O-Si bond formation between ceria abrasive grains and silica substrate, hydrolysis reaction of Si-O bond, and associated mono-silica dissociation from the substrate. We also performed simulations of systems containing chemical compounds added to the slurry for the purpose of controlling the polishing rate and observed the role of additives on the CMP process. In this presentation, we will show simulation results and discuss the mechanism of CMP of silica substrates with ceria abrasive grains in terms of dependence on surface morphology at the atomic scale and the role of additives. Reference [1] Tran*, R., Lan*, J., Shuaibi*, M., Wood*, B., Goyal*, S., Das, A., Heras-Domingo, J., Kolluru, A., Rizvi, A., Shoghi, N., Sriram, A., Ulissi, Z., & Zitnick, C. (2022). The Open Catalyst 2022 (OC22) Dataset and Challenges for Oxide Electrocatalysis. arXiv preprint arXiv:2206.08917.

Angstrom surface on copper induced by novel green chemical mechanical polishing using ceria and silica composite abrasives

Toxic and corrosive slurries are widely employed in traditional chemical mechanical polishing (CMP), leading to the pollution to the environment. Consequently, it is a challenge to acquire angstrom surface using green CMP. To solve this challenge, a novel green CMP was developed for copper (Cu), including only four ingredients. Ceria and silica are used as composite abrasives. Tartaric acid was applied to adjust the pH value of the developed green slurry to 6.5. After CMP, surface roughness Ra is 0.1 nm measured by atomic force microscopy and surface profilometer respectively, with measurement areas of 5 × 5 μm2 and 50 × 50 μm2, correspondingly. So far, this is the lowest surface roughness reported in such a large measurement area. Transmission electron microscopy confirms that the thickness of damaged layer after CMP varies from 0.4 to 2 nm. CMP mechanism is elucidated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Firstly, the Cu surface was oxidized by hydrogen peroxide, then solved via hydrogen ions, and finally chelated with hydroxyl groups of tartaric acid. These findings provide a novel approach and new insights to achieve angstrom level surface, for the potential use in high-performance devices of Cu.

Atomic surface of fused silica and polishing mechanism interpreted by molecular dynamics and density functional theory

For fused silica, it is difficult to obtain an atomic surface with a high material removal rate (MRR), and the polishing mechanism is still a challenge. Chemical mechanical polishing (CMP) has historically used toxic and polluted ingredients. To meet these demanding constraints, it is extremely difficult to produce green CMP for fused silica. In this work, we report on the development of a novel green CMP slurry containing ceria, nicotinic acid, sodium alginate, and deionized water. After CMP, an atomic surface is achieved with a surface roughness Sa of 0.098 nm using a scanning area of 20 × 20 μm2, and the MRR is 7.65 μm/h. According to density functional theory and molecular dynamics simulations, nicotinic acid and fused silica generated a novel complex during CMP that could be peeled by active ceria by creating coordinating bonds. These findings offer an intuitive understanding of the CMP mechanism and suggest a novel method for fabricating fused silica atomic surfaces for possible usage in high-performance optical devices.

Effect of potassium persulfate on chemical mechanical planarization of Cu/Ni microstructures for MEMS

Electrochemical FABrication (EFAB) can fabricate complex metal microstructures. Planarization is crucial in EFAB in terms of flatness and surface quality. Accordingly, chemical mechanical polishing (CMP) was employed to planarize Cu/Ni microstructures. The oxidant K2S2O8 is critical in regulating the material removal rate (MRR) selectivity between copper and nickel. In the presence of the complexing agent ethylenediamine sulfate and corrosion inhibitors benzotriazole and sodium dodecyl benzene sulfonate, the MRR selectivity of 1 (∼47 nm/min) and satisfactory surface quality can be achieved by tuning the K2S2O8 concentration to 20 mM. A two-step planarization method was proposed. After lapping for 7 min and CMP for 21 min, the step height of the Cu/Ni microstructures rapidly decreases from 251 μm to −46 nm on average, and the surface roughness Sa of copper and nickel are significantly improved to 8.7 nm and 2.9 nm, respectively. For the CMP mechanism, as the K2S2O8 concentration increases, the copper corrosion is enhanced, and thus the copper MRR quickly increases; The nickel surface film gradually transforms from Ni(OH)2 to NiOOH, and the intermediate state can be removed relatively easily, and thereby the nickel MRR increases and decreases. This study may provide a feasible high-precision planarization method for EFAB.

Chemical mechanical polishing of silicon wafers using developed uniformly dispersed colloidal silica in slurry

It is a challenge to enhance the material removal rate (MRR) during the chemical mechanical polishing (CMP) of silicon wafers, with increasing the size of a wafer and reducing the feature size of integrated circuit. In this study, potassium ions were employed to improve the MRR of silicon wafers effectively in CMP, which was performed by developed slurry, consisting of monodisperse alkaline colloidal silica, organic/inorganic acids and inorganic salts. Experimental results show that the developed slurry has a good stability and dispersity for colloidal silica. The MRR of silicon wafers in CMP was increased by 53.42 % to 1778.71 Å/min, when the concentration of potassium ions was 125 mmol/L. In addition, the surface roughness Sa was also improved slightly to 0.887 nm. The CMP mechanism was investigated using X-ray photoelectron spectroscopy, friction coefficient meter and electrochemical analysis. It reveals that potassium ions could change the electrical characteristics of the formed soft layer on the silicon wafer surface, dissolving the soft layer and oxidizing the surface of silicon.

Effect of a water film on the material removal behavior of Invar during chemical mechanical polishing

Understanding polishing mechanisms in water-lubricated environments has an important guiding value for surface engineering of precision devices. This work reveals the chemical mechanical polishing (CMP) mechanism of the Invar alloy under water lubrication by use of molecular dynamics simulation. The results show that the appropriate thickness of the water film and polishing speed can significantly reduce the surface roughness of the work piece and eliminate subsurface defects. With the increasing rolling speed, the variation of the surface roughness and subsurface damage thickness exhibits a trend of slowly decreasing and then reaching stability. In addition, with an increase in water film thickness, more defects were formed in the subsurface region due to greater surface stresses, although the roughness could be reduced due to an increase of passivated atoms on the work piece surface. These results would be helpful for understanding the CMP mechanism under water-lubricated conditions and for promoting the development of surface engineering for micro/nano components.

Atomic-scale surface of fused silica induced by chemical mechanical polishing with controlled size spherical ceria abrasives

Fused silica is widely used in precision optical devices for its excellent optical properties. However, it has hard and brittle nature, becoming a difficult-to-process material. This makes microscopic cracks left on the processed surface of silica induced by traditional mechanical processing methods. In this regard, it is a challenge to achieve an atomic-scale surface on silica. To overcome this challenge, nanometer CeO2 abrasives with uniform shape and size were prepared via a hydrothermal method. A novel chemical mechanical polishing (CMP) slurry is developed using the prepared CeO2 abrasives, Na2CO3 and deionized water, which is low-cost, non-toxic, and environmentally friendly. The surface roughness, Sa of fused silica after CMP is 0.093 nm with a scanning area of 20 × 20 μm2. To the best of our knowledge, it is the lowest surface roughness on silica after CMP. The CMP mechanism was investigated by X-ray photoelectron spectroscopy and infrared Fourier transformation. Under alkaline conditions, the abrasives interacted with the silica forming a CeOSi reaction product, through a chemical tooth action, which is significant to produce an atomic-scale surface on silica. These findings provide a new pathway to fabricate atomic-scale surface for a hard-brittle solid.

Environment-friendly chemical mechanical polishing for copper with atomic surface confirmed by transmission electron microscopy

Atomic and close-to-atomic scale manufacturing is a new technology that can achieve atomic-level manufacturing precision and functional critical dimension. Here we propose an efficient approach for atomic-scale chemical mechanical polishing (CMP) surface of fine copper utilizing optimized CMP slurry and polishing parameters. The optimal polishing slurry is composed of silica, hydrogen peroxide, aspartic acid, sodium phosphate and deionized water, where abrasive particles of silica nanospheres synthesized are uniformly dispersed. Prior to CMP, copper was precisely lapped by ceria slurry. After CMP, the surface roughness is 0.099 nm with an area of 50 × 50 µm2. Atomic-level flatness surface is acquired. To the best of our knowledge, it is the lowest surface roughness with such a relatively large scanning area, compared with those published previously. High resolution transmission electron microscopy (HRTEM) images of polishing surface further prove that atomic-scale surface roughness is achieved. X-ray photoelectron spectra clarify CMP mechanism that complex oxidation, ionization and chelation are sparked on copper surface and the soft layer is formed. The presented efficient approach for atomic-scale polishing will widen the application fields of copper materials and copper-based components.

Improving Chemical Mechanical Polishing Efficiency of PZT with Less than 100 ppm SO4 2−

Excellent surface quality of lead zirconate titanate (PZT) is required to achieve high performance in micro-electro-mechanical systems. In this work, PZT-4 (Pb(Zr0.44Ti0.56)O3) was processed with chemical mechanical polishing (CMP). K2SO4 was used to enhance the CMP efficiency. The results reveal that at pH 4 and 6, as the K2SO4 concentration increases, the material removal rate (MRR) and surface roughness S a first increase, then decrease and stabilize. Interestingly, at pH 6, after adding only 55 ppm SO4 2−, the MRR increases by 54%. After CMP, the surface becomes smooth. No visible processing damage occurs in the substrate. For the CMP mechanism, SO4 2− in K2SO4 plays an essential role in the MRR enhancement. At pH 6, PbO can dissolve, forming oxygen vacancies. After adding a low concentration of SO4 2−, the negatively charged SO4 2− ions can be attracted by the positively charged oxygen vacancies. Then soluble Zr(SO4)2 can be formed, promoting the dissolution of Zr4+. As a result, the surface integrity can be destroyed. Therefore, the MRR is improved. However, with a high concentration of SO4 2−, the PbSO4 precipitates can inhibit dissolution and wear. Hence, the MRR is suppressed. This research may provide a feasible approach for high-efficiency CMP of PZT.

A novel visible-light catalyzed assisted single crystal diamond chemical mechanical polishing slurry and polishing mechanism

Single crystal diamond has significant chemical and mechanical stability, which makes it challenging to efficiently obtain high quality single crystal diamond surface. In this study, the effect of four different sets of chemical mechanical polishing (CMP) slurries on the surface roughness Ra and the material removal rate MRR was investigated. The optimum slurry consisted of hydrogen peroxide, diamond micro-powder, synthesis product of nanodiamond and CuFe-layered double hydroxide (ND/LDH), oxalic acid and deionized water. After CMP by the optimum slurry, the average Ra value of three scanning ranges of 868 µm× 868 µm was 0.112 ± 0.003 nm and the average MRR value of three measurements was 416 ± 5 nm/h. High efficiency to obtain high quality single crystal diamond surface was achieved. The variation of hydroxyl radicals (·OH) concentration in the optimum slurry with time was measured. The optimum slurry CMP mechanism was elucidated by X-ray diffractometry (XRD) and X-ray photoelectron spectrometer (XPS). Visible-light photoelectrons and the rapid oxidation of C2O4·- radicals were accelerated the redox cycle of Fe3+/Fe2+ and Cu2+/Cu+ to produce·OH. The·OH oxidised the C atoms on the single crystal diamond surface to form -C-O-C-, -C-O-H and -CO. Finally, they were removed by diamond micro-powder abrasive, obtaining an ultra-smooth surface.

Surface acidity of colloidal silica and its correlation with sapphire surface polishing

Colloidal silica occupies the field of polishing solution application of chemical mechanical polishing (CMP). Si-OH groups dominate the surface of colloidal silica nanoparticles, and their chemical nature, acidity, is a determining factor for CMP. However, due to the delicate nature, very vulnerable to self-condensation at elevated temperatures, of colloidal silica nanoparticles, traditional techniques used for characterizing powder samples are limited to gaining true insights into the chemical reactivity of these Si-OH groups without disturbing the surface chemistry, which hinders the deep exploration of the essential mechanism of CMP on silica. In this manuscript, low-field nuclear magnetic resonance (LF NMR) and freeze-drying-programmed temperature desorption (TPD) techniques were, for the first time, combined to investigate the true acidity of the surface Si-OH groups in colloidal silica with minimized perturbance from sample preparations. The results from both characterization methodologies complemented each other, and are in good agreement about the surface structure and acidity of the surface Si-OH groups on colloidal silica. The results demonstrate that the acidity of Si-OH hydroxyl groups on the surface of silica-soluble nanoparticles spreads in a very broad range from very weak, medium-strong, to very strong. CMP performance of sapphire substrates by using colloidal silica strongly correlates with the acidic nature of surface Si-OH groups at the acidic polishing condition. The stronger acidity of surface Si-OH groups, the higher material removal rates in sapphire polishing through the chemical bonding of deprotonated Si-O- functional surface of colloidal silica with hydrolyzed sapphire Al-OH surface. Therefore, when a large number of Si-O- active centers in the basic environment attack the sapphire Al-OH sites, the rate of the chemical reaction is accelerated.

Atom-by-Atom and Sheet-by-Sheet Chemical Mechanical Polishing of Diamond Assisted by OH Radicals: A Tight-Binding Quantum Chemical Molecular Dynamics Simulation Study.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

Two-Step Chemical Mechanical Polishing of 4H-SiC (0001) Wafer

Chemical mechanical polishing (CMP), as a widely used global smoothing technique, requires a high polishing rate and a low surface roughness. However, it is difficult to meet these requirements with single-step polishing. To obtain a perfect 4H-SiC wafer surface, we studied a two-step CMP 4H-SiC wafer using a-Al2O3-based slurry and nano-SiO2-based slurry. Before the two-step process was determined, the effect of concentration of KMnO4 and pH on the polishing rate of 4H-SiC was first studied with Al2O3 as the abrasive, and the maximum polishing rate of 1400 nm h−1 was determined under the condition of pH of 2.00 and KMnO4 concentration of 6.5 wt% in the step. Furthermore, the effect of H2O2 concentration on the polishing rate of 4H-SiC was investigated using SiO2 as the abrasive and V2O5 as the catalyst. It was determined that under the condition of V2O5 concentration of 4 wt% and H2O2 concentration of 10 wt%, the maximum polishing rate of the second step polishing was 150 nm h−1, and a perfect surface was obtained, with a surface roughness of 0.066 nm. In addition, the in situ friction coefficients have used to characterize the two-step process. At the end, the CMP mechanisms of the above two slurry have been deduced and preliminarily demonstrated.

A novel slurry for chemical mechanical polishing of single crystal diamond

Single crystal diamond (SCD) has extremely high hardness and wear resistance. However, this hampers the development of surfaces with ultra-low surface roughness and ultra-thin damage layer. It is particularly challenging to achieve the chemical mechanical polishing (CMP) of SCD with a surface roughness less than 0.5 nm and a damage layer thickness <1 nm. In this study, the influence of the CMP slurries with different pH values and oxidants on the quality of polished SCD surface was investigated. The optimal slurry consisted of silica, ferrous sulfate, hydrogen peroxide, nitrilotriacetic acid, and deionized water achieved the lowest surface roughness of SCD. The average surface roughness Ra was 0.754 ± 0.062 nm over eight areas of 868 μm × 868 μm. The Ra value of 0.35 nm was obtained over a scanning area of 70 μm × 50 μm, while the thickness of the damage layer was 0.7 nm. The CMP mechanism was elucidated by X-ray photoelectron spectroscopy and infrared spectroscopy. Hydroxyl radicals and hydrogen ions were adsorbed on the surface, oxidation of the surface and generated C-H, C-O, and C = O groups, and they are eventually removed by silica abrasive, yielding an ultra-smooth SCD surface.

Non-spherical abrasives with ordered mesoporous structures for chemical mechanical polishing

The chemical mechanical polishing (CMP) technology has been widely used for surface modification of critical materials and components with high quality and efficiency. In a typical CMP process, the mechanical properties of abrasives play a vital role in obtaining the ultra-precision and damage-free surface of wafers for improvement of their performances. In this work, a series of fine structured rod-shaped silica (RmSiO2)-based abrasives with controllable sizes and diverse ordered mesoporous structures were synthesized via a soft template approach, and successfully applied in the sustainable polishing slurry for improving the surface quality of cadmium zinc telluride (CZT) wafers. Compared with commercial silica gel, solid and mesoporous silica spheres, the RmSiO2 abrasives present superior elastic deformation capacity and surface precision machinability on account of their mesoporous structures and rod shapes. Especially, ultra-precision surface roughness and relatively effective material removal speed were achieved by the CMP process using the RmSiO2 abrasives with a length/diameter (L/d) ratio of 1. In addition, a potential CMP mechanism of the developed polishing slurry to CZT wafer was elucidated by analyzing X-ray photoelectron spectra and other characterizations. The proposed interfacial chemical and mechanical effects will provide a new strategy for improving abrasives’ machinability and precision manufacture of hard-to-machine materials.

Chemical mechanical polishing for sapphire wafers using a developed slurry

At present, corrosive ingredients are widely employed in chemical mechanical polishing (CMP) of sapphire, bringing a potential threat to both people and environment. This results in high cost and long processing time on the treatment of CMP slurry of sapphire. It is a challenge to develop a novel green CMP for sapphire to meet the stringent requirements of high performance products. In this study, a novel green CMP is proposed for sapphire, consisting of silica nanoparticles of 50 nm, triethanolamine (TEA), sodium metasilicate nonahydrate, and deionized water. After green CMP, surface roughness Ra, root mean square (rms), and peak-to-valley (PV) are 0.11, 0.139, and 1.65 nm, respectively. Material removal rate is 3.31 μm/h during green CMP. To the best of our knowledge, surface roughness Ra and rms after green CMP developed are the lowest on a sapphire wafer, compared with those reported previously. The CMP mechanism is elucidated by X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectra. Sapphire formed Al(OH) 4 − ions in an alkaline environment, which was chelated by TEA, and removed from the surface of sapphire. These findings provide new insights to fabricate high performance devices of sapphire for the use in semiconductor and microelectronic industries.

First-principles molecular dynamics investigation of ceria/silica sliding interface toward functional materials design for chemical mechanical polishing process

We report a form of chemical mechanical polishing (CMP) in which ceria abrasive particles polish a silicon wafer surface in a water environment. The Car-Parrinello molecular dynamics (CPMD) method, which enables chemical reaction dynamics in non-equilibrium systems to be treated non-empirically, was used. The first application of CPMD was to a model including a silica surface rubbed by a ceria cluster to ascertain the fundamental tribochemical phenomena in the CMP process. The model consisted of a hydrogen-terminated silica surface (a representative model of an oxidized wafer surface) and H4Ce6O12 cluster placed in the vicinity of an asperity in the silica surface. H2O molecules were explicitly considered in order to reflect the presence of slurry water. An external load and shear force were applied to the cluster. Under conditions of finite temperature and friction, a Ce-O-Si bridging bond formed at the interface between the silica surface and the ceria cluster. Subsequently, another Ce-O-Si bridging bond formed. These newly formed bridging bonds changed the silicon atom into a five-coordinated one. Because this state was probably unstable, a pyramid structure was rapidly restored by breaking the original Si-O bond in the asperity, meaning that the wafer surface became flattened. By referring to static density functional theory (DFT) calculations, we concluded that this bond dissociation event was due to multiple Ce-O-Si bonds forming. To discuss the effect of slurry water, a similar CPMD simulation, but without H2O molecules (i.e., dry friction), was performed. Here, the Si-O bond dissociation reaction did not take place even if multiple Ce-O-Si bonds formed. This means slurry water is needed for CMP to occur and it probably helps to weaken the Si-O bond. The second application of CPMD was to a model consisting of a thin water layer sandwiched between silica and ceria plates for studying the effect of the surface orientation of the ceria. Two low-index CeO2 surfaces, (100) and (111), were modeled. A chemical event wherein Ce-O-Si bridging bonds formed on the ceria/silica interface could be observed in both models. However, the bond dissociation reaction of the original Si-O bond on the silica surface proceeded only in the model with the (111) surface, and not on the (100) surface. This difference in reactivity could mainly be explained by the arrangement of atoms of each surface. The (111) surface provides hexagonal adsorption sites that inherently form a structure with five-coordinated silicon atoms, which are important intermediates in the CMP process. On the other hand, the (100) surface exhibits square adsorption sites which hardly form the required intermediate. We successfully examined the effect of the surface orientation of the ceria in addition to the fundamental CMP mechanism wherein multiple Ce-O-Si bonds initiate a chemical reaction including Si-O bond scission.

Recent developments and applications of chemical mechanical polishing

This article discusses advanced developments and applications of chemical mechanical polishing (CMP) published recently in the selected papers indexed by Web of Science. The topics covered are advances in slurry and abrasives, pads and conditioning, CMP for semiconductor device manufacturing, CMP for other applications, modeling and simulations, and CMP with ultrasonic vibrations, lasers, photocatalysts, or UV lamps. Nonspherical abrasive particles have been developed for CMP, resulting in increased material removal rates (MRRs). Advanced conditioning methods have been proposed to uniformly generate pad surface shapes. Fixed abrasive CMP has advantages with higher MRRs. New models for designing the pad and conditioner have been proposed, and more uniform pad shapes can be obtained. Integrated advanced process control improves the wafer-to-wafer variation. Dental implants treated by CMP perform equally or better than the baseline-machined implants and the biphasic calcium phosphate-treated implants. The slurry distribution and the abrasive behavior can be simulated by means of multiphase modeling. Molecular dynamics simulations can explore the mechanism of CMP. CMP of wafers can be simulated using an atomic force microscope with its tapping mode. Theoretical models have been developed to calculate removal depths, study the chemical action in CMP, and explore the crystal orientation effects. CMP assisted by ultrasonic vibrations increases CMP MRRs and lowers the roughness of polished surfaces. CMP assisted by using UV lamp power, femtosecond lasers, or photocatalysts enhances CMP MRRs.