Catalyst-referred Etching Research Articles

High-Efficiency Polishing of Polymer Surface Using Catalyst-Referred Etching

Previously, we developed an abrasive-free polishing technique called catalyst-referred etching (CARE) for inorganic materials. In this method, the topmost site of the workpiece surface is preferentially removed via an indirect hydrolysis reaction promoted by a metal catalyst. In this study, we proposed applying the CARE method to polymer material polishing and demonstrated the polishing characteristics. Using the CARE method, polycarbonate, which has an easy cleavage of ester bond via hydrolysis, was polished, resulting in the smoothness of the surface roughness below 1.0 nm. Based on the surface observations, the removal mechanism was estimated as follows. Molecule chains are entangled to form clusters constituting the polymer surface and help determine the surface roughness. In the CARE method, the top of this cluster was selectively removed, thus creating a smooth surface. Polymers with C–C bonds, such as polymethyl methacrylate and fluorinated ethylene propylene, were also smoothed using the CARE method. These results indicate that the CARE method is highly effective in polishing polymer materials.

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.

Bias-assisted photoelectrochemical planarization of GaN (0001) with impurity concentration distribution

To planarize semiconductor materials such as gallium nitride (GaN) and silicon carbide with high efficiency, we developed a polarization method that combines ultraviolet irradiation and an abrasive-free polishing method known as catalyst-referred etching (CARE). In this method, the substrate surface is photoelectrochemically oxidized, thus improving the removal rate. Accordingly, an atomically well-ordered surface was obtained at a removal rate 100 times higher than that of the conventional CARE method without ultraviolet irradiation. However, in some cases, for GaN substrates with a high oxygen impurity concentration area, the oxidation rate is nonuniform on the substrate surface, resulting in the formation of a rough surface. In this study, we propose the application of a positive bias to the GaN substrate to suppress the oxidation rate fluctuation. In the positive bias state, the width of the depletion layer generated at the interface of GaN and the etchant becomes uniform on the entire surface regardless of crystallographic fluctuation, thereby achieving a uniform oxidation rate. When only 3.0 V was applied, the oxidation rate was uniform; thus, a flat GaN surface without the footprint originating from crystallographic fluctuations was obtained.

Role of Photoelectrochemical Oxidation in Enabling High-Efficiency Polishing of Gallium Nitride

Photoelectrochemical (PEC) oxidation is currently employed to improve the removal rates of surface polishing methods applied to gallium nitride (GaN) wafers, such as PEC mechanical polishing (PECMP) and PEC catalyst-referred etching (PEC-CARE). The use of PEC along with these methods has successfully achieved removal rates as high as 720 nm h−1 while still ensuring the highly ordered step-and-terrace structures generated by the same methods when PEC-oxidation is not employed. However, the atomic-scale phenomena induced by PEC oxidation to assist the polishing process of GaN surfaces remain unclear. To address such a question, this study investigates the role of PEC oxidation during CARE as an example. The results demonstrate that PEC oxidation selectively occurs at the step edge and advances through the GaN (0001) surface laterally rather than in the depth direction, thus enabling high-speed polishing and resulting in a highly ordered surface, both crystallographically and geometrically.

Polishing Approaches at Atomic and Close-to-Atomic Scale

Roughness down to atomic and close-to-atomic scale is receiving an increasing attention in recent studies of manufacturing development, which can be realized by high-precision polishing processes. This review presents polishing approaches at atomic and close-to-atomic scale on planar and curved surfaces, including chemical mechanical polishing, plasma-assisted polishing, catalyst-referred etching, bonnet polishing, elastic emission machining, ion beam figuring, magnetorheological finishing, and fluid jet polishing. These polishing approaches are discussed in detail in terms of removal mechanisms, polishing systems, and industrial applications. The authors also offer perspectives for future studies to address existing and potential challenges and promote technological progress.

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.

Planarization of Lithium Niobate Surface Using a Thin Film Catalyst in Pure Water

A catalytically assisted etching method, named Catalyst-Referred Etching (CARE) was applied to the planarization of Lithium Niobate (LN) surface, which is widely used for optical waveguides, optical modulators, piezoelectric applications. The study demonstrates that an atomically smooth surface with less than 0.1 nm root-mean-square roughness could be achieved on a LN substrate using a thin metal film and pure water as the catalyst and etching solution, respectively. All residual stress and surface damage could be removed completely thanks to the removal mechanism of CARE.

Photoelectrochemical Oxidation Assisted Catalyst-Referred Etching for SiC (0001) Surface

In a previous study, we developed an abrasive-free polishing method named catalyst-referred etching (CARE) and used it for the planarization of silicon carbide (SiC) (0001). In this method, Si atoms at step edges are preferentially removed through a catalytically assisted hydrolysis reaction to obtain an atomically smooth and crystallographically well-ordered surface. However, the removal rate is low (< nm/h) and needs to be improved. In this study, we proposed an ultraviolet (UV) light assisted CARE method. In this method, UV light is irradiated onto a SiC surface to generate holes and oxidize the surface. The oxidized area, consisting of SiO2, can be quickly removed to form a nano-pit owing to the higher removal rate of SiO2 compared to that of SiC. The periphery of the nano-pits works as a reaction site, leading to a higher removal rate. To enhance the oxidation rate and form nano-pits, we applied electrochemical bias to the SiC substrate. However, the removal rate did not improve significantly when the bias voltage was higher than 3.0 V. This is because the electrochemical potential of Pt increased with the anodic potential of SiC, which oxidized the Pt surface and degraded the catalyst capability. To avoid this issue, we modified the catalytic pad, where an in-situ refreshment of the Pt surface is possible. As a result, the removal rate increased up to 200 nm/h at a bias of 7.0 V, which is 100 times higher than that of the CARE without UV irradiation. The proposed method is expected to contribute to the enhancement in the productivity and quality of next-generation SiC substrates.

High surface laser-induced damage threshold of SrB4O7 single crystals under 266-nm (DUV) laser irradiation.

Under 266-nm (deep ultraviolet, DUV) laser irradiation, an SrB4O7 (SBO) single crystal has been found to exhibit a surface laser-induced damage threshold (LIDT) of ∼ 16.4 J/cm2, which is higher than those of a synthetic silica glass (4.8 J/cm2) and a calcium fluoride (CaF2) crystal (11.4 J/cm2). By catalyst-referred etching (CARE), the LIDT of an SBO crystal can also be improved to around 24.1 J/cm2, which is 1.4 and 6.0 times higher compared to an unetched crystal and a silica glass, respectively. With high surface LIDTs, SBO single crystals can then be used as optical window materials for high-power DUV laser systems.

An abrasive-free chemical polishing method assisted by nickel catalyst generated by in situ electrochemical plating

An abrasive-free polishing method using water and a Pt catalyst, called catalyst-referred etching (CARE), has been developed for the finishing of optical and semiconductor surfaces. This method realizes well-ordered surfaces with a smoothness of several tens of picometers without crystallographic disturbance. In this study, we propose a new CARE method using a Ni catalyst with in situ electrochemical plating and dissolution, which enable enhancing the catalytic capability of Ni. This method has advantages to realize more than ten times higher removal rate and better stability compared with the conventional CARE method.

High-Efficiency Planarization of SiC Wafers by Water-CARE (Catalyst-Referred Etching) Employing Photoelectrochemical Oxidation

Catalyst-referred etching (CARE) is an abrasive-free and damage-free polishing method that involves applying a catalytic reaction at the contact point of the catalyst surface and workpiece in a chemical solution. An atomically flat silicon carbide (SiC) wafer surface can be obtained by the CARE process. Recently, it was found that water can be used as a chemical solution, even in the case of SiC polishing. However, its current removal rate of 4H-SiC (0001) 4°off-axis substrate is only 2 nm/h and is expected to increase. In this study, the use of photoelectrochemical oxidation in combination with the CARE process using water was investigated, successfully increasing the removal rate up to approximately 100 nm/h.

Characteristics and Mechanism of Catalyst-Referred Etching Method: Application to 4H-SiC

A novel abrasive-free planarization method named catalyst-referred etching (CARE) was developed. A polishing pad is coated with a catalytic material to promote chemical etching of the work substrate. During processing, the topmost areas of the work substrate, which are in contact with the catalyst surface, are selectively etched off. Atomically highly ordered surfaces are obtained for many types of work substrates. In this paper, the removal characteristics and mechanism of CARE for single crystalline 4H-SiC are reviewed.

High-efficiency planarization of GaN by catalyst-referred etching using photo-electrochemical oxidation

High-efficiency planarization of GaN by catalyst-referred etching using photo-electrochemical oxidation

触媒表面基準エッチング法による GaN 基板平坦化における加工速度の高速化

触媒表面基準エッチング法による GaN 基板平坦化における加工速度の高速化

Planarization of 6-Inch 4H-SiC Wafer Using Catalyst-Referred Etching

Catalyst-referred etching (CARE) is a planarization method based on the chemical etching reaction, which does not need abrasives. In this paper, CARE was applied to the planarization of 6-inch silicon carbide (SiC) wafers, and removal properties were investigated. The etching rate was about 20nm/h, which is almost equal to that of 2-inch SiC wafer (16 nm/h). The rms roughness was reduced along with the removal depth, and step-terrace structure was observed in whole area of the on-axis wafer surface.

Improvement of I-V Characteristics of Schottky Barrier Diode by 4H-SiC Surface Planarization

Leakage current points in 4H-SiC Schottky barrier diodes (SBDs) were found to be in the same location as rough surface morphologies. Rough surfaces after activation annealing at the Schottky interface of diodes were removed by chemical mechanical polishing (CMP) or catalyst-referred etching (CARE). As a result, this study succeeded in reducing the leakage current and barrier height variation of 1.2 kV SBDs by eliminating surface roughness.

Smoothing single-crystalline SiC surfaces by reactive ion etching using pure NF3 and NF3/Ar mixture gas plasmas

In pure NF3 plasma, the etching rates of four kinds of single-crystalline SiC wafer etched at NF3 pressure of 2 Pa were the highest and it decreased with an increase in NF3 pressure. On the other hand, they increased with an increase in radio frequency (RF) power and were the highest at RF power of 200 W. A smooth surface was obtained on the single-crystalline 4H-SiC after reactive ion etching at NF3/Ar gas pressure of 2 Pa and addition of Ar to NF3 plasma increased the smoothness of SiC surface. Scanning electron microscopy observation revealed that the number of pillars decreased with an increase in the Ar-concentration in the NF3/Ar mixture gas. The roughness factor (Ra) values were decreased from 51.5 nm to 25.5 nm for the As-cut SiC, from 0.25 nm to 0.20 nm for the Epi-SiC, from 5.0 nm to 0.7 nm for the Si-face mirror-polished SiC, and from 0.20 nm to 0.16 nm for the C-face mirror-polished SiC by adding 60% Ar to the NF3 gas. Both the Ra values of the Epi- and the C-face mirror-polished wafer surfaces etched using the NF3/Ar (40:60) plasma were similar to that treated with mirror polishing, so-called the Catalyst-Referred Etching (CARE) method, with which the lowest roughness of surface was obtained among the chemical mirror polishing methods. Etching duration for smoothing the single-crystalline SiC surface using its treatment was one third of that with the CARE method.

Investigation of the Barrier Heights for Dissociative Adsorption of HF on SiC Surfaces in the Catalyst-Referred Etching Process

We have developed a novel abrasive-free planarization method, which we term catalyst-referred etching (CARE). In silicon carbide (SiC) CARE, Pt is used as a catalyst and HF solution is used as an etchant. CARE produces a crystallographically undamaged and smooth SiC surface. To understand the removal mechanism at the topmost surface of SiC in the CARE process, we performed first-principles reaction path simulations using the simulation tool for atom technology (STATE) program package. These calculations are based on the density functional theory within the generalized gradient approximation of Perdew et al. The barrier height of the dissociative adsorption of HF on a SiC surface was evaluated by the climbing image nudged elastic band method. We present simulation results for the initial stages of the etching process. The reaction barrier height for adsorption of the first HF is 1.2 eV.

Planarization of the Gallium Nitride Substrate Grown by the Na Flux Method Applying Catalyst-Referred Etching

In this study, a gallium nitride (GaN) substrate grown by the Na flux method was processed by catalyst-referred etching (CARE). CARE process for GaN involves two-step: the first step uses the photoelectrochemical process (PEC-CARE) and the second step applies a platinum (Pt) catalyst (Pt-CARE). PEC-CARE can produce a highly ordered GaN (0001) surface. The surface roughness was 0.638 nm root-mean-square (rms). Pt-CARE can produce an atomically flat surface with a step-terrace structure on the entire surface. The surface roughness was 0.113 nm rms. The removal rates of PEC-CARE and Pt-CARE were found to be 45.5 and 5.5 nm/h, respectively.

4H-SiC Planarization Using Catalyst-Referred Etching with Pure Water

A novel abrasive-free polishing method called catalyst-referred etching (CARE) has been developed. CARE can be used to chemically planarize a silicon carbide (SiC) surface with an etching agent activated by a catalyst. Platinum (Pt) and hydrofluoric (HF) acid are used as the catalyst and etchant, respectively. CARE can produce an atomically flat and crystallographically highly ordered surface of 4HSiC (0001) with a root-mean-square roughness of less than 0.1 nm regardless of the cut-off angle. However, industrial use of CARE is difficult because of HF acid usage. In this study, pure water was investigated as an alternative etchant to HF acid. We examined CARE using pure water by applying it to the planarization of a 4HSiC substrate and observed a feasible performance. The removal mechanism is considered to be the dissociative adsorption of water molecules to the SiC bonds of the topmost Si atom, namely the hydrolysis of the back bond, and the catalysis of Pt is considered to enhance the reaction. CARE with pure water is expected to represent a breakthrough method for surface processing of SiC, and will be widely applied in industrial processes such as planarization after high temperature processing in device fabrication.