Traditional Chemical Mechanical Polishing Research Articles

Atomic surface of silicon wafers induced by grafted silica nanoparticles and sodium carbonate

Atomic surface of silicon (Si) wafers is highly desired for integrated circuit (IC) industry. To meet this demand, traditional chemical mechanical polishing usually employs noxious slurries with complex compositions, containing oxidant, dispersant, inhibitor, surfactant, etc., resulting in the pollution of the environment and high-cost post-treatments. To settle this challenge, a novel green chemical mechanical polishing was developed, only including grafted silica nanoparticles and sodium carbonate. After CMP, surface roughness Sa of 0.219 and 0.247 nm are achieved on Si wafers using amine and methyl modified nanoparticles, respectively, corresponding to material removal rate (MRR) of 132.53 and 84.82 nm/min. Amino-modified nanoparticles increase 18.2 % MRR, and methyl modified ones reduce 24.4 % MRR, compared with those of unmodified nanoparticles. X-ray photoelectron spectroscopy reveals that amino-grafted nanoparticles have strong improvement for chemical reactions during CMP. Density functional theory calculations garner the electrostatic potential energy of 4.884 and 6.753 eV for amine and methyl-modified nanoparticles, respectively, corresponding to high and low effects for MRR. This work provides a new avenue to develop novel green simple CMP for Si wafers, reducing the pollution and cost for post-treatments.

Novel green chemical mechanical polishing by controlling pH values and redox reaction for achieving atomic surface of a nickel alloy

Traditional chemical mechanical polishing (CMP) normally employs noxious slurries, leading to pollution to the environment. It is a challenge to achieve atomic surfaces of nickel (Ni) alloy using green CMP. To overcome this challenge, a novel green CMP was developed, including composite abrasives of ceria and silica, mandelic acid, hydrogen peroxide and deionized water. Using the developed green CMP, atomic surface with surface roughness Sa of 0.179 nm was achieved, under a scanning area of 50 × 50 μm2. Electrochemical tests indicate that the developed slurry has effective corrosion ability on Ni alloy. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy reveal that the Ni alloy was oxidized by hydrogen peroxide, forming oxides of Ni, chromium (Cr) and molybdenum (Mo). Cr and Mo oxides demonstrated better stability in mandelic acid, while Ni oxides reacted with hydrogen ions and produced Ni2+ and Ni3+ ions. The ions chelated with mandelic acid. Finally, the generated oxides and reaction products were removed by abrasives and a polishing pad under chemical and mechanical balanced functions. The developed green CMP paves a new avenue to obtain atomic surface of a Ni alloy for the possible applications of high-performance components in aerospace industry.

Angstrom surface with high material removal rate for quartz glass induced by silk dissolved novel green chemical mechanical polishing

Quality and efficiency are a pair of everlasting contradiction for manufacturing, and therefore it is a challenge to achieve an atomic surface with high material removal rate (MRR). Toxic and corrosive slurries are widely employed in traditional chemical mechanical polishing (CMP), facing challenges to treat the detrimental slurries during and after CMP. To solve these challenges, natural extract of sodium alginate (SA) and ceria are used to develop green CMP. Dissolved silk is firstly applied to shear thickening and enhance MRR in CMP. After CMP, surface roughness Sa is 0.08nm on the surface of quartz glass at a scanning area of 20×20 μm2, and MRR is 33.215 μm/h, and thickness of the damaged layer is 5.09nm. To the best of our knowledge, the surface roughness is the lowest and MRR is the highest for quartz glass, compared with those reported previously. Density functional theory and molecular dynamics simulations reveal that dissolved silk is adsorbed onto the surface of quartz glass, forming a complex with SA and facilitating the removal of surface atoms. Additionally, the developed novel green slurry of CMP demonstrates bio-compatibility and degradability. These outcomes suggest new viewpoints that using a dissolved silk slurry of ceria breaks the eternal contradiction between efficacy and surface roughness to achieve atomic surface with high MRR via green CMP.

Atomic-scale planarization surface of quartz glass induced by novel green chemical mechanical polishing using three ingredients

Traditional chemical mechanical polishing (CMP) usually employs more than three kinds of toxic or corrosive ingredients for its slurry. Quartz glass is a hard-brittle solid, and it is extremely difficult to achieve its atomic-scale planarization surface. In these respects, it is a challenge to obtain atomic surface of quartz glass using green CMP slurry with three ingredients. To solve this challenge, a novel green CMP was developed, containing three green ingredients, such as ceria, sodium alginate and aminobutyric acid. After CMP, atomic surface is acquired with surface roughness Ra of 0.093 nm and material removal rate of 12.47 μm/h. Transmission electron microscopy confirms the thickness of damaged layer is 8.57 nm. X-ray photoelectron spectroscopy shows that –OH, C–N and –COOH groups adsorbed on the surface of quartz glass after immersion in CMP slurry, which agrees well with the simulated results of molecular dynamics. Density functional theory calculations reveal that electron transfer groups of aminobutyric acid and sodium alginate were predominantly located on –NH2 and –COOH, forming the covalent bonds between them. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy indicate that new complexes were generated between aminobutyric acid and sodium alginate, increasing the viscosity of CMP slurry. Furthermore, the breakage of C–N and O–H bonds after CMP suggests that the complexes play a pivotal role in the CMP process. The developed novel green CMP with three ingredients provides new viewpoints to obtain atomic-scale planarization surface for a hard-brittle solid.

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.

Atomic surface achieved through a novel cross-scale model from macroscale to nanoscale.

Chemical mechanical polishing (CMP) is widely used to achieve an atomic surface globally, yet its cross-scale polishing mechanisms are elusive. Moreover, traditional CMP normally employs toxic and corrosive slurries, resulting in potential pollution to the environment. To overcome these challenges, a novel cross-scale model from the millimeter to nanometer scale is proposed, which was confirmed by a newly developed green CMP process. The developed CMP slurry consisted of hydrogen peroxide, sodium carbonate, sodium hydroxycellulose, and silica. Prior to CMP, fused silica was polished by a ceria slurry. After CMP, the surface roughness (Sa) was 0.126 nm, the material-removal rate was 88.3 nm min-1, and the thickness of the damaged layer was 8.8 nm. The proposed model was built by fibers, through integrating Eulerian and Lagrangian models and reactive force field-molecular dynamics. The results predicted by the model were in good agreement with those of CMP experimentally. A model for large-sized fibers revealed that a direct contact area of 11.12% was obtained for a non-woven polishing pad during the CMP experiments. Another model constructed via combining Eulerian and Lagrangian functions showed that the stress at the intersections of the fibers varied mainly from 0.1 to 0.01 MPa and was higher than the stress at other parts. An increase in viscosity led to a decrease in the areas with low stress, demonstrating that viscosity enhanced the stress and facilitated the removal of material. At the microscale and nanoscale, the stress of the abrasive surface exposed to the workpiece changed from 2.21 to 6.43 GPa. Stress at the interface contributed to the formation of bridging bonds, further promoting the removal of material. With increasing the compressive stress, the material-removal form was transformed from a single atom to molecular chains. The proposed model and developed green CMP offer new insights to understand the cross-scale polishing mechanism, as well as for designing and manufacturing novel polishing slurries, pads, and setups.

Atomic surface of quartz glass induced by photocatalytic green chemical mechanical polishing using the developed SiO2@TiO2 core-shell slurry.

High-performance devices of quartz glass demand an atomic surface, which induces a challenge for chemical mechanical polishing (CMP) with a high material removal rate (MRR). Moreover, traditional CMP usually employs toxic and corrosive slurries, leading to the pollution of the environment. To overcome these challenges, a novel green photocatalytic CMP is proposed. In the CMP, SiO2@TiO2 core-shell abrasives were developed, and the CMP slurry included the developed abrasives, sodium carbonate, hydrogen peroxide and sorbitol. After photocatalytic CMP, the surface roughness Sa of quartz glass is 0.185 nm, with a scanning area of 50 × 50 μm2, and the MRR is 8.64 μm h-1. To the best of our knowledge, the MRR is the highest on such a big area of atomic surface for quartz glass. X-ray photoelectron spectroscopy reveals that SiO2@TiO2 core-shell abrasives were used as photocatalysts motivated by simulated solar light, generating electrons and holes and producing hydroxyl radicals through hydrogen peroxide. As a result, OH- could combine with Si atoms on the surface of quartz glass, forming Si-OH-Si bonds. Then the formed bonds were removed based on the balance between chemical and mechanical functions. The proposed CMP, developed SiO2@TiO2 abrasives and slurry provide new insights to achieve an atomic surface of quartz glass with a high MRR.

Novel green chemical mechanical polishing for an aluminum alloy and mechanisms interpreted by molecular dynamics simulations and measurements

Aluminum (Al) alloy is a kind of soft-plastic metal, and traditional chemical mechanical polishing (CMP) is easy to produce scratches on the polished surface and embed of abrasives, resulting in the degradation of devices. In addition, conventional CMP generally employs toxic and corrosive slurries, leading to the pollution of environment. To address these challenges, a novel green CMP was developed for Al alloy, consisting of ceria, silica, sodium carbonate, hydrogen peroxide, glycine and deionized water. After CMP, surface roughness Sa is 0.258 nm under an area of 100 × 100 μm2, which is the lowest surface roughness reported previously in such a big measurement area for Al alloy. Transmission electron microscopy confirms that the thickness of damaged layer is merely 4.6 nm. X-ray photoelectron microscopy and Fourier transform infrared spectroscopy reveal that composite abrasives are conducive for the generation of Al-O, Al-O-Si and Ce-Al bonds on the polished surface. Molecular dynamics simulations elucidate that the subsurface damage is mainly caused by exerted loading of abrasives on amorphous layer, as well as vacancy induced through the annihilation of dislocations profoundly. These outcomes provide a new avenue to manufacture atomic surfaces of soft-plastic metals for latent applications in devices.

Recent Advances in Chemical Mechanical Polishing Technologies of Silicon Carbide

Silicon Carbide (SiC) as the third generation semiconductor material has a very high surface quality requirement, which is the core index in the engineering application. Chemical Mechanical Polishing (CMP) is the only technology that can realize global flattening and non-damage surfaces at present. In this paper the recent advances in traditional CMP, the effects of slurry, and hybrid CMP of SiC were reviewed. The principles and recent developments of CMP were introduced. Then the influence of various factors of slurry on polishing performance was discussed. In addition, recent advances in hybrid CMP technologies, such as Electrochemical Mechanical Polishing (ECMP), Photocatalytic Chemical Mechanical Polishing (PCMP), Plasma Assisted Polishing (PAP), Catalyst-Referred Etching (CARE) were reported, especially some efforts to make these technologies environmentally friendly. Finally, some shortcomings of CMP technology in the application of SiC are summarized and prospect its future development trends.

Insight into Polishing Slurry and Material Removal Mechanism of Photoassisted Chemical Mechanical Polishing of YAG Crystals.

Yttrium aluminum garnet (YAG) crystals are an important gain medium in thin-sheet solid-state lasers, and their processing quality directly affects the performance of solid-state lasers. But it is difficult to achieve high efficiency and high quality of YAG crystals by traditional chemical mechanical polishing (CMP). In this study, we developed a new polishing slurry for photoassisted chemical mechanical polishing (PCMP) of YAG crystals. The polishing slurry is composed of peroxymonosulfate (PMS), manganese ferrite (MnFe2O4), alumina (Al2O3) abrasives, and deionized water. PCMP is conducted in an ultraviolet (UV) light environment. When employing this polishing slurry for PCMP processing of YAG crystals, the material removal rate (MRR) achieved 250 nm/min and the surface roughness achieved 0.35 nm Sa. The experiments verified that both UV light and MnFe2O4 can effectively activate PMS to produce active free radicals and further enhance the chemical action of the polishing slurry. X-ray photoelectron spectroscopy (XPS) analysis results indicated that active radicals reacted with the surface structure of the crystal and removed the aluminum-oxygen octahedron in large quantities from it. The structural defects reduced the surface hardness of the crystal, which means that active free radicals can modify the crystal surface materials.

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.

Novel green chemical mechanical polishing of fused silica through designing synergistic CeO2/h-BN abrasives with lubricity

It is a challenge to produce an atomic-scale surface of fused silica with a high material removal rate (MRR) using green chemical mechanical polishing (CMP). In traditional CMP, a slurry usually contains four or more components and is normally toxic and corrosive, causing environmental degradation. To address this problem, a novel green CMP slurry for fused silica was developed using ceria/hexagonal boron nitride (CeO2/h-BN) abrasives, potassium oleate, and deionized water. Due to the two-dimensional nanosheet structure of h-BN, it is simple to generate a sheared layer, which lowers friction at the contact area. Ceria abrasives realized a new function for superlubricity during CMP through this design, preventing damages and reaching atomic-scale surface. This is confirmed by research showing that a damaged layer’s thickness following CMP, as determined by high-resolution transmission electron microscopy, is only 2.7 nm. The MRR of fused silica is 31.92 µm•h−1 during CMP, and surface roughness Sa is 0.124 nm after CMP. The MRR is currently at its highest with such a low surface roughness. These findings provide a new pathway to achieve an atomic-scale surface for a hard-brittle material with a high MRR using the synergistic effect of abrasives.

CmpCNN: CMP Modeling with Transfer Learning CNN Architecture

Performing chemical mechanical polishing (CMP) modeling for physical verification on an integrated circuit (IC) chip is vital to minimize its manufacturing yield loss. Traditional CMP models calculate post-CMP topography height of the IC’s layout based on physical principles and empirical experiments, which is computationally costly and time-consuming. In this work, we propose a CmpCNN framework based on convolutional neural networks (CNNs) with a transfer learning method to accelerate the CMP modeling process. It utilizes a multi-input strategy by feeding the binary image of layout and its density into our CNN-based model to extract features more efficiently. The transfer learning method is adopted to different CMP process parameters and different categories of circuits to further improve its prediction accuracy and convergence speed. Experimental results show that our CmpCNN framework achieves a competitive root mean square error ( RMSE ) of 2.7733Å with 1.89× reduction compared to the prior work, and a 57× speedup compared to the commercial CMP simulation tool.

The mechanical effect of soft pad on copper chemical mechanical polishing

Smaller transistor size challenges traditional chemical mechanical polishing (CMP) in semiconductor fabrication. Novel consumables and in-depth mechanism research can support defect mitigation and improve yield. This study focused on soft pad polishing of copper film from the perspective of mechanical effects. To reduce the defects, a soft pad is replacing the hard pad in the barrier layer polishing and over-polishing steps of copper CMP. When the contact behavior of pad asperity and wafer was in the elastic-plastic regime, the exponential influence factors of pressure, abrasive concentration, and sliding velocity on the material removal rate (MRR) were around 0.64, 0.22, and 0.64, respectively. Stick-slip friction with squeal noise scratched the copper film at the beginning and end of the CMP process. The pad conditioning removed polishing residues but deflected the pad asperity. Proper conditioning frequency is necessary to optimize the polishing process.

Effects of Chemical-Electrical and Mechanical Parameters on Electrical-induced Chemical Mechanical Polishing of GaN

Overcoming the low fabricating efficiency of traditional chemical mechanical polishing (CMP) for Gallium nitride (GaN) is a challenge owing to its high hardness, high brittleness, and chemical inertness. Here, electrochemical etching is proposed to increase the material removal rate and acquire a high-quality surface on GaN wafers. To reveal the synergistic etching mechanism of oxidizing agent and corrosion inhibitor on the GaN wafers, electrochemical etching experiments were carried out. The optimal etching solution contained 4 wt% H2O2 and 10 mmol l−1 purified terephthalic acid. Experiments with various polishing parameters were comparatively investigated to verify the auxiliary effect of etching and determine the ideal parameters. Cathodoluminescence spectroscopy shows that the electrochemical etching removes the SSDs completely and the CMP process with befitting parameters does not induce supernumerary SSDs.

Sulfuric Acid Etching for Surface Modification and Improved Polishing of Magnesium Aluminate Spinel

Magnesium aluminate spinel is a material with high optical transparency and great potential for optical applications. However, its high hardness, high strength and chemical inertness make obtaining the required surface quality quickly through traditional processing a challenge. Existing assisted machining methods are complicated and costly, which limits their application to high-efficiency and ultra-precision manufacturing. In this study, etching was employed as a polishing preprocess owing to its obvious advantages of simplicity, practicality and low cost. Sulfuric acid was applied to etching spinel, and the influences of the etching temperature and time on the surface morphology and mechanical properties of the spinel were evaluated. Surface analytical techniques were performed to clarify the mechanism for improved polishing. Experimental results indicated that sulfuric acid etching significantly reduced the surface hardness. The surface quality and efficiency were significantly improved with the etching pretreatment compared with traditional chemical mechanical polishing. Thus, the proposed method of sulfuric acid etching-assisted polishing can reduce the difficulty of spinel machining to realize the high-efficiency and precision manufacturing for a wider range of applications.

Improvement in chemical mechanical polishing of 4H-SiC wafer by activating persulfate through the synergistic effect of UV and TiO2

Chemical mechanical polishing (CMP) has revolutionized the processing of semiconductor material such as silicon carbide (SiC). However, the strong chemical bond between silicon atoms and carbon atoms gives SiC materials a very high mechanical hardness and chemical stability. Therefore, SiC wafers may suffer from a low material removal rate (MRR) and serious subsurface damage when traditional CMP methods are used. In this study, to improve the CMP properties of Si-face of the 4H-SiC wafers, the synergistic effect of UV and TiO 2 on the activation of persulfate (PS) was investigated for the first time. The response surface method was employed to study the effect of pH, PS concentration, and TiO 2 dosage on CMP in-depth, and to ultimately optimize the polishing process. The analysis demonstrated that an MRR of 608 nm/h can be obtained with the average surface roughness ( Sq ) of 0.521 nm when the pH is 6, in which the TiO 2 dosage is 0.015 wt% and the PS concentration is 3 wt%. Polarization curves, AC impedance spectra and X-ray photoelectron spectroscopy tests were performed to further analyze the chemical action of the slurry, which showed that the introduction of UV-TiO 2 strongly enhances the chemical action of slurry and results in the formation of a passivated layer on the SiC surface. Finally, based on the radical quenching experiments and the results mentioned above, a model that relates to the removal of 4H-SiC by UV-TiO 2 synergistic activation of PS is proposed.

Orthogonal Experimental Research on Dielectrophoresis Polishing (DEPP) of Silicon Wafer.

Silicon wafer with high surface quality is widely used as substrate materials in the fields of micromachines and microelectronics, so a high-efficiency and high-quality polishing method is urgently needed to meet its large demand. In this paper, a dielectrophoresis polishing (DEPP) method was proposed, which applied a non-uniform electric field to the polishing area to slow down the throw-out effect of centrifugal force, thereby achieving high-efficiency and high-quality polishing of silicon wafers. The principle of DEPP was described. Orthogonal experiments on important polishing process parameters were carried out. Contrast polishing experiments of silicon wafer were conducted. The orthogonal experimental results showed that the influence ratio of electric field intensity and rotation speed on material removal rate (MRR) and surface roughness was more than 80%. The optimal combination of process parameters was electric field intensity 450 V/mm, rotation speed 90 rpm, abrasive concentration 30 wt%, size of abrasive particle 80 nm. Contrast polishing experiments indicated that the MRR and material removal uniformity of DEPP were significantly better than traditional chemical mechanical polishing (CMP). Compared with the traditional CMP, the MRR of DEPP was increased by 17.6%, and the final surface roughness of silicon wafer reached Ra 0.31 nm. DEPP can achieve high-efficiency and high-quality processing of silicon wafer.

Thin Ferrite Wafer Uniformly Polished with Magnetic Attraction Method

When a ferrite substrate is used to prepare thin-film InSb Hall element chips, one surface of the ferrite wafer needs to be polished. The traditional chemical mechanical polishing (CMP) method, often used for polishing of ferrite wafers, has several problems: a long time to install and remove wafers, a little high debris rate, and high cost. Taking advantage of the fact that ferrite wafers can be magnetically attracted, a method using independent RuFeB magnets to attract and hold ferrite wafers is proposed. The polishing head holding the ferrite wafer can perform planetary rotary motion to achieve uniform polishing. The polishing head floats up and down freely on the polishing pad, without the requirements of matching and process accuracy in machining, mechanical linkage, and control. The roughness of the ferrite surface after polishing is about 0.18 μm, which meets the requirements for the substrate of InSb Hall element chips. With no debris, the efficiency of installing and removing wafers is 30 times higher than that of traditional methods. At the same time, compared with traditional CMP machine, the cost of CMP equipment made by this method is very low.

Polishing single-crystal silicon carbide with porous structure diamond and graphene-TiO2 slurries

In this study, a self-modified diamond (SMD) slurry for mechanical polishing is presented as well as a novel TiO2-graphene slurry used with the aid of UV light for a chemical mechanical polishing (CMP) technique that improves the rate of polishing. The surface characteristics and removal rate of silicon carbide (SiC) samples polished with the slurry were compared with the results obtained by polishing with both a conventional diamond slurry and CMP slurry. The experimental results indicated that the material removal rate (MRR) of SiC from the wafer surface with the SMD slurry was higher, and surface finish and lapping efficiency was better than that achievable with other slurries. The experimental data also showed that a TiO2-graphene slurry had a higher removal rate compared to the TiO2 slurry using the same intensity of UV light. The TiO2-graphene slurry showed a 3-fold increase in removal rate over that of a traditional CMP slurry. An experiment was also carried out where nanodiamond (50 nm and 500 nm) abrasive was added to the graphene-TiO2. As expected, the removal rate increased but the average surface roughness was poor because the nanodiamond particles caused small scratches. The graphene-TiO2 with nanodiamond abrasives has 5 times the removal rate achieved with a traditional CMP slurry. This novel method can facilitate the extensive production of well-polished single-crystal SiC wafers.