Electrochemical Mechanical Polishing Research Articles

Sustainable Electrochemical Mechanical Polishing (ECMP) for 4H-SiC wafer using chemical-free polishing slurry with hydrocarbon-based solid polymer electrolyte

Large-diameter single-crystal silicon carbide (SiC) wafers with good surface quality are desirable to dramatically improve the performance of SiC-based power electronic devices. However, the preparation of such wafers using a strong oxidative polishing slurry has the limitations of low material removal rate (MRR) and high environmental impact. Electrochemical–mechanical polishing (ECMP) initiates surface removal by electrochemical oxidation without using harsh chemicals, making it an alternative with low environmental impact to conventional polishing. Herein, we propose the use of polystyrene sulfonic acid (PSS), a hydrocarbon-based polymer electrolyte without perfluoro compounds, for ECMP. The SiC surface was effectively oxidized by ECMP using PSS. The effects of the polyelectrolyte crosslinking treatment on the lifetime of polyelectrolyte-incorporated nonwoven cloths were investigated. The optimum electrolytic and mechanical conditions yielded high MRRs of 37 and 14 µm/h for 2- and 4-inch SiC wafers. Moreover, the surface morphologies of the growth face and nonfacet areas was discussed. The wafer-scale evaluation demonstrated the roughness reduction to 0.85 nm Sa (average) after 15-min ECMP of the 4-inch wafer. The polishing-induced residual stress was investigated using microscale and wafer-scale Raman mapping. Overall, the proposed ECMP process offers significant advantages in terms of speed, cost-effectiveness, and environmental impact.

On the Post‐Processing of Complex Additive Manufactured Metallic Parts: A Review

Additive manufacturing (AM) is gaining more attention due to its capability to produce customized and complex geometries. However, one significant drawback of AM is the rough surface finish of the as‐built parts, necessitating post‐processing for achieving the desired surface quality that meets application requirements. Post‐processing of complex geometries, such as parts with internal holes, lattice structures, and free‐form surfaces, poses unique challenges compared to other components. This review classifies various post‐processing methods employed for complex AM parts, presenting the experimental conditions for each treatment alongside the resulting improvement in surface roughness as a success criterion. The post‐processing methods are categorized into four groups: electrochemical polishing (ECP), chemical polishing (CP), mechanical polishing, and hybrid methods. Notably, mechanical methods exhibit the highest roughness improvement at 69.9%, followed by ECP (59.9%), hybrid methods (47.4%), and CP (49.5%). Nevertheless, mechanical post‐processing techniques are less frequently utilized for lattice parts, making chemical or electrochemical methods more promising alternatives. In summary, all four categories of post‐processing methods can improve the internal surfaces quality of AM holes. While mechanical methods offer the most substantial roughness improvement overall, chemical and electrochemical methods show particular potential for addressing the challenges associated with complex geometries.

Investigation of electrochemical behavior and polishing mechanism in electrochemical mechanical polishing of cobalt

Investigation of electrochemical behavior and polishing mechanism in electrochemical mechanical polishing of cobalt

ЭКСПЕРИМЕНТАЛЬНЫЕ ИССЛЕДОВАНИЯ КОМБИНИРОВАННОГО ПРОЦЕССА ЭЛЕКТРОХИМИКОМЕХАНИЧЕСКОГО ПОЛИРОВАНИЯ

This article presents the results of experimental studies of the electrochemical mechanical polishing process, which combines anodic dissolution, possessing high localizing properties, with the dynamic effects of abrasive grains. The essence of the combined process is described, when, under the influence of operating voltage, a continuous electrochemical dissolution of the treated surface occurs with the formation of oxide films on it, followed by their removal together with the treated metal by means of abrasive grains quasi-fixed in the polishing pad. The description and conditions of the conducted experiments are given. The influence of the weight content of the abrasive in the composition of the electrolyte-abrasive mixture on the removal of metal and the roughness of the treated surface is shown. The impact on the course of the combined process of the frequency of rotation of the polishing pad and the pressure of the workpiece on it is noted. At optimal values of the studied parameters, electrochemical-mechanical polishing makes it possible to ensure high geometric quality of the treated surface and high process productivity.

Effects of electrolyte type and concentration on the anodic oxidation of 4H-SiC (0001) in slurryless electrochemical mechanical polishing

Slurryless electrochemical mechanical polishing (ECMP) is a promising polishing method for SiC wafers. To achieve a high polishing efficiency, low surface roughness, low cost, and low environmental load, the effects of the electrolyte type and concentration on the anodic oxidation and ECMP of the 4H-SiC (0001) surface were investigated. It is confirmed that the anodic oxidation rate is mainly affected by the conductivity but not the type of electrolyte, and the electric conductivity affects the oxidation current that determines the anodic oxidation rate and performance. The anodic oxidation rate of the SiC surface was restricted by the conductivity at low electrolyte concentrations, and the anodic oxidation rate increased with increasing electrolyte concentration in this stage. The anodic oxidation rate decreased with increasing electrolyte concentration after the conductivity exceeded ∼0.1 S/m, owing to the increasing resistance of the generated oxide layer. Overall, the anodic oxidation uniformity of the SiC surface increased with increasing electrolyte concentration owing to the increasing anodic oxidation current density. The changes in material removal rate of ECMP with increasing electrolyte concentration were consistent with those of the anodic oxidation rate, while the surface roughness obtained by ECMP always decreased with increasing electrolyte concentration. This study provides a new way to improve the performance of slurryless ECMP.

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.

Selective electrochemical mechanical polishing of 4H–SiC surface employing porous material impregnated with electrolyte

Slurryless electrochemical mechanical polishing (ECMP) is a high-efficiency and high-precision method that can be applied to SiC wafers. However, this method has not been used on curved surfaces because it is difficult to realize selective ECMP due to electrolyte liquidity. In this study, a novel selective ECMP method using porous material impregnated with electrolyte was proposed, and its feasibility was verified using porous quartz glass. In addition, the effects of porous material pore size, applied voltage, current density, and oxidation time on the selective anodic oxidation performance were investigated and clarified. A porous material with a larger pore size exhibited a higher anodic oxidation rate, but the oxidation area was more difficult to control. A large electrical field and the hydrophilicity of the oxide layer facilitated electrolyte exudation from the porous material. Moreover, selective ECMP was conducted on the 4H–SiC (0001) surface, porous quartz glass showed poor polishing performance and could not remove the oxidized layer. Selective ECMP using a ceria grinding wheel obtained a material removal rate of 4.5 μm/h and significantly decreased the surface roughness from 79.219 to 2.166 nm Sq in 20 min, showing great application potential. This study provided a novel polishing method for difficult-to-machine curved surfaces.

Slurryless electrochemical mechanical polishing of 4-inch 4H–SiC (0001) and (000–1) surfaces

Silicon carbide (SiC) is a very promising next-generation semiconductor material for power device applications, and polishing technology plays an important role in improving the performance of electronics. In this regard, slurryless electrochemical mechanical polishing (ECMP) is a promising polishing technology for SiC wafer in obtaining high efficiency and high precision. In this study, slurryless ECMP of 4-inch 4H–SiC (0001) and (000–1) surfaces (Si and C faces) were investigated aiming to the industrial application. Anodic oxidation property, polishing characteristics, and mechanism of the Si and C faces in slurryless ECMP were compared and clarified. Based on the obtained results, two-step slurryless ECMP (i.e., ECMP with diamond polishing plate and then with ceria polishing plate) was conducted on 4-inch as-sliced Si and C faces. Additionally, 4-inch Si and C faces with peak-to-valley (PV) value less than 2 μm and overall surface roughness in nanometer order were obtained.

Environment-friendly electrochemical mechanical polishing using solid polymer electrolyte/CeO2 composite pad for highly efficient finishing of 4H-SiC (0001) surface

Silicon carbide (SiC) is a promising material for use in high-performance power electronic devices. However, the SiC wafer is costly partly due to difficulties in machining the SiC surface. Furthermore, its prolonged polishing process requires massive amounts of resources, which increases the environmental impact of the wafer production process. Herein, we developed an environment-friendly and highly efficient electrochemical mechanical polishing (ECMP) method for 4H-SiC (0001) that does not require any liquid containing harsh chemicals and uses an ion-conductive composite pad consisting of a solid polymer electrolyte and CeO2 particles. The SiC surface finished using the proposed method was analyzed using X-ray analysis, Raman spectroscopy, atomic force microscopy, and X-ray adsorption fine structure measurements. A high material removal rate (MRR) of approximately 15 µm h−1, which is almost 10 times higher than that obtained by CMP, was achieved by the proposed ECMP method under the current density and mechanical condition of 25 mA cm−2 and 2.3 × 103 kPa min−1, respectively. The proposed method, which can remove surface damage and produce a smooth surface on the entire wafer scale with a high MRR without a liquid electrolyte, should prove to be advantageous for environment-friendly SiC wafer preparation.

A new polishing method for complex structural parts: Moist particle electrolyte electrochemical mechanical polishing (MPE-ECMP)

The improvement of the surface quality of complex structural parts is a major challenge. Electrochemical mechanical polishing (ECMP) plays an important role in the polishing of wafers due to its high material removal rate. However, electrochemical mechanical polishing of the surface of complex structures, such as the bevels of turbine blades, has been little investigated. This paper proposes a new method, moist particle electrolyte electrochemical mechanical polishing (MPE-ECMP), for polishing the surfaces of complex structural parts. Using a sulfonated cation exchange resin as a moist particle electrolyte (MPE), polarization curves are measured using linear scanning voltammetry (LSV) and a selected potential of 1.0 V vs. Hg|Hg2SO4 over a 1 h polishing period. The elemental changes on the copper surface are examined by energy dispersive spectroscopy (EDS) and those on the MPE surface are studied using X-ray photoelectron spectrometry (XPS) to explain the material removal mechanism. The results show that the surface roughness of copper is reduced from the initial Sa = 433.48 nm to Sa = 21.83 nm, a reduction of 95.0%, that no copper oxides are generated during the MPE-ECMP process and that the MPE adsorbs released Cu2+. It is demonstrated that MPE-ECMP has good surface finishing properties for beveled surfaces, and provides an effective approach for surface polishing of complex structural parts.

Experimental study on plasma enhanced electrochemical mechanical polishing of SiC single crystal

SiC single crystal is a good semiconductor material with high breakdown electric field strength, high thermal conductivity, high carrier saturation drift speed and low relative dielectric. However, SiC has high hardness and brittleness, high chemical stability, so the polishing efficiency of existing polishing methods is low. In this paper, a plasma enhanced electrochemical mechanical polishing method for SiC was proposed. The relationship between peak voltage, frequency and duty ratio with polishing efficiency and surface roughness was studied.

Improvement in the polishing characteristics of titanium-based materials using electrochemical mechanical polishing

Titanium (Ti) and its alloys are widely used in aerospace components and biomaterials. The smoothing of Ti materials is essential for realizing high-quality Ti-based devices and components to enhance their physical and chemical properties, such as fatigue strength and biocompatibility. However, with conventional polishing methods, obtaining sub-nanometer roughness in a large measurement area is difficult because of the generation of crystal grain steps on the polished surface. This paper presents an electrochemical mechanical polishing (ECMP) method that involves electrochemical oxidation and subsequent oxide removal using SiO2 particles to achieve a high material removal rate (MRR) and an ultra-smooth surface of pure Ti and Ti-6Al-4V. Under suitable electrolytic current and mechanical conditions, ECMP provides a higher MRR and smoother pure Ti surface than electroless polishing. X-ray and Raman analyses indicate that the mechanical condition affects the chemical states of polished surfaces; higher mechanical conditions retain less oxide film on the polished surface, resulting in a smoother surface. The highly efficient ECMP provides a defect-free and sub-nanometer roughness for pure Ti and Ti-6Al-4V surfaces without using chemicals. Furthermore, it is an environment-friendly and cost-effective approach to prepare Ti-based components.

Damage-free polishing of SiC by Slurryless Electrochemical Mechanical Polishing

Damage-free polishing of SiC by Slurryless Electrochemical Mechanical Polishing

Charge Utilization Efficiency and Side Reactions in the Electrochemical Mechanical Polishing of 4H-SiC (0001)

Slurryless electrochemical mechanical polishing (ECMP) is very effective in the polishing of silicon carbide (SiC) wafers. To achieve a high material removal rate (MRR) of SiC wafer using ECMP with low electrical energy loss, charge utilization efficiency in the anodic oxidation of the SiC surface was investigated and the underlying mechanism was clarified by modeling the anodic oxidation system of SiC in 1 wt% NaCl aqueous solution. The charge utilization efficiency in the anodic oxidation of SiC was found to be constant when the current density was less than 20 mA cm−2 and significantly decreased when the current density was greater than 30 mA cm−2, resulting in a significant reduction in the MRR. Modeling of the anodic oxidation system indicates that the charge utilization efficiency depended on the potential applied on the SiC surface: the oxidation of SiC occupied the dominant position in the anodizing system when the potential is lower than 25 V vs Ag∣AgCl, charge utilization efficiency greatly decreased when the applied potential was greater than 25 V owing to the occurrence of oxidations of the H2O and Cl−. This research provides both a theoretical and practical foundation for using ECMP to polish SiC wafers.

A Review on Precision Polishing Technology of Single-Crystal SiC

Single-crystal SiC is a typical third-generation semiconductor power-device material because of its excellent electronic and thermal properties. An ultrasmooth surface with atomic surface roughness that is scratch free and subsurface damage (SSD) free is indispensable before its application. As the last process to reduce the surface roughness and remove surface defects, precision polishing of single-crystal SiC is essential. In this paper, precision polishing technologies for 4H-SiC and 6H-SiC, which are the most commonly used polytypes of single-crystal SiC, such as chemical mechanical polishing (CMP), photocatalytic chemical mechanical polishing (PCMP), plasma-assisted polishing (PAP), electrochemical mechanical polishing (ECMP), and catalyst-referred etching (CARE), were reviewed and compared with emphasis on the experimental setup, polishing mechanism, material removal rate (MRR), and surface roughness. An atomically smooth surface without SSD can be obtained by CMP, PCMP, PAP, and CARE for single-crystal SiC. However, their MRRs are meager, and the waste treatment after CMP is difficult and expensive. Moreover, PAP’s operation is poor due to the complex polishing system, plasma generation, and irradiation devices. A high MRR can be achieved by ECMP. In addition, it is an environmentally friendly precision polishing process for single-crystal SiC since the neutral salt solution is generally used as the electrolyte in ECMP. However, the formation of the egglike protrusions at the oxide/SiC interface during anodic oxidation would lead to a bigger surface roughness after ECMP than that after PAP is processed. The HF solution used in CARE was toxic, and Pt was particularly expensive. Ultrasonic vibration-assisted single-crystal SiC polishing and electrolyte plasma polishing (EPP) were discussed; furthermore, the research direction of further improving the surface quality and MRR of single-crystal SiC was prospected.

Efficient and slurryless ultrasonic vibration assisted electrochemical mechanical polishing for 4H–SiC wafers

This paper proposes a slurryless, highly efficient polishing method called ultrasonic vibration assisted electrochemical mechanical polishing (UAECMP) to realize 4H–SiC wafers with subnanometer surface roughness. UAECMP involves using ultrasonic vibration to simultaneously assist anodic oxidation of the SiC surface and mechanical removal of the generated oxide layer. The performance of UAECMP was evaluated by experiments and theoretical analyses. For a 4H–SiC (0001) surface, UAECMP achieved a material removal rate (MRR) of 14.54 μm/h, which was 4.5 times greater than that of ordinary electrochemical mechanical polishing (ECMP) and 290 times greater than that of mechanical polishing. Ultrasonic vibration increased the anodic oxidation rate by introducing local transient strain to the SiC surface and increasing the temperatures of the polishing area and electrolyte. The effect increased with the amplitude of the ultrasonic vibration. However, increasing the ultrasonic vibration amplitude also increased the surface roughness due to the large fluctuations of polishing marks caused by the grinding stone and SiC surface impact and the increasing residual oxide. Therefore, we propose a high-efficiency and -quality polishing process for SiC wafers that combines UAECMP and ECMP. The proposed polishing process may help simplify the existing manufacturing process for SiC wafers.

High-efficiency wafer-scale finishing of 4H-SiC (0001) surface using chemical-free electrochemical mechanical method with a solid polymer electrolyte

Silicon carbide (SiC) has been extensively studied for applications in next-generation high-power electronic devices. High-quality and low-cost electronic devices require a surface finishing process that can produce smooth and defect-free 4H-SiC (0001) surfaces. However, SiC surfaces are difficult to remove because of their extremely high mechanical strength and chemical stability. Herein, we present a high-efficiency, wafer-scale, chemical-free finishing process for a 4H-SiC (0001) surface using electrochemical mechanical polishing (ECMP) with a solid polymer electrolyte (SPE). The ECMP method comprises electrochemical oxidation at the SiC/SPE interface and subsequent oxide removal by CeO2 particles. Electrolysis with a SiC (anode)/SPE/cathode electrochemical system demonstrated that the use of SPE as an alternative to liquid electrolytes allows the efficient electrochemical oxidation of highly inert SiC (0001) surfaces without requiring any chemicals. The ECMP process achieved a high material removal rate for SiC (0001) in the range of 1.8–9.2 μm/h, although strongly dependent on a 50 to 450 mA electrolytic current. Moreover, an electrolytic current of 250 mA produced a smooth and defect-free surface with a sub-nanometer-scale roughness (0.68 nm Sa) and excellent uniformity over an entire 2-inch SiC (0001) surface. Atomic force microscopy observations showed that ECMP under a low electrolytic current (30 mA) combined with subsequent CeO2 polishing without electrolysis can help improve microscale surface morphologies. The proposed ECMP, which produces a smooth and uniform surface without requiring chemicals, is a highly efficient and environmentally friendly finishing process for the fabrication of SiC wafers.

Development of polystyrene/polyaniline/ceria (PS/PANI/CeO2) multi-component abrasives for photochemical mechanical polishing/planarization applications

Tri-layer nanostructured polystyrene/polyaniline/ceria (PS/PANI/CeO2) multi-component composites were developed as functionalized particle abrasives for improved photochemical mechanical polishing/planarization (PCMP) applications. The physicochemical characteristics of the PS/PANI/CeO2 and PS/CeO2 samples were examined by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Flourier-transform infrared spectroscopy, UV–visible spectrophotometer, photoluminescence spectroscopy, and thermogravimetric analysis. Ultra-smooth and defect-free surfaces with ultra-low roughness (0.12 nm Ra, 0.16 nm RMS) were achieved after PCMP with polymer/CeO2 hybrid abrasives. It might be attributed to the enlarged contact area and the decreased indention depth between the particle and the surface, possibly originating from the elastic polymer cores. The synergy of the PANI conductive polymer and the CeO2 particle photocatalyst contributed to improving the separation efficiency of photo-generated carriers, thereby enhancing the photochemical oxidation activity. As expected, the tri-layer PS/PANI/CeO2 abrasives achieved a ca. 69% increment of material removal rate compared to the PS/CeO2. The interfacial contact model and material modification/removal mechanism of the PS/PANI/CeO2 multi-component tri-layer abrasives during PCMP processes were also discussed. The aim of this work is expected to provide guidance for the design and promising applications of novel polymer/CeO2 composite abrasives in advanced CMP, PCMP, electrochemical mechanical polishing (ECMP), and photoelectrochemically combined mechanical polishing (PECMP).

Correction to: Electrochemical polishing versus mechanical polishing of AISI 304: surface and electrochemical study

Correction to: Electrochemical polishing versus mechanical polishing of AISI 304: surface and electrochemical study

Electrochemical polishing versus mechanical polishing of AISI 304: surface and electrochemical study

Electrochemical polishing versus mechanical polishing of AISI 304: surface and electrochemical study