Abrasive-free Polishing 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.

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.

Study on rheological properties and polishing performance of viscoelastic material for dilatancy pad

Shear dilatancy polishing (SDP) is an emerging flat polishing method for hard and brittle materials, which can achieve high-efficiency and high-precision polishing of workpieces under high-pressure and high-speed conditions. In our research, a novel viscoelastic material (VM) with non-Newtonian fluid properties was prepared. The dilatancy pad was subsequently obtained based on VM, abrasives, and a polyurethane pad. The effects of abrasive size and abrasive concentration on the rheological properties of VM were analyzed. The mechanism of abrasives in VM was investigated, and the principle of SDP was revealed. VM-30 wt%Diamond (0.5 μm) with excellent shear hardening performance was selected in the polishing experiment to study the effects of polishing time, polishing speed, and polishing pressure on polishing performance, and the optimal polishing parameters of SDP for tungsten were determined. Compared with traditional abrasive free polishing (AFP), SDP can realize a higher material removal rate (MRR increased by 33.4%) and better surface quality (Sa reduced by 32.2%) due to the shear dilatancy effect under high-pressure and high-speed conditions. After 90 min of SDP, the surface roughness (Sa) of tungsten workpiece was reduced from 307.9 nm to 12.1 nm. Results show that SDP is a promising and feasible polishing method for hard and brittle materials.

Novel abrasive-free jet polishing for Bulk single-crystal KDP with a low viscosity microemulsion

In present work, the abrasive-free jet polishing (AFJP) of bulk single-crystal KDP was first fulfilled, when using a newly-designed low-viscosity microemulsion as the AFJP fluid. The novel AFJP fluid shows a typical water-in-oil structure, in which the water cores uniformly distribute in the BmimPF6 IL, with a particle size of about 20–25 nm. What’s more, the AFJP fluid is a controllable and selective non-abrasive jet fluid that the shape of the removal function is regular and smooth, presenting a similar Gaussian function, meanwhile, the dispersion coefficient of the removal rate is only 1.9%. Finally, the surface quality of the bulk single-crystal KDP is further improved by AFJP, meanwhile, the subsurface damage is first obviously mitigated.

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.

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.

Abrasive-free surface finishing of glass using a Ce film

A novel form of abrasive-free polishing is described, using Ce film deposited on a polishing pad by vacuum evaporation. Polishing using metallic films other than Ce, such as Al and Cu, generates defects on the glass surface with almost negligible material removal rates (MRRs), whereas Ce film with deionized (DI) water achieves smooth, scratch-free surfaces with noticeable MRR. If DI water is not supplied to the pad, there is significant deterioration in polishing performance. MRR increases with greater thickness of Ce film, and extremely smooth surface with sub-nanometer roughness is achieved using Ce film of thickness 2.5 μm. Polishing performance is dependent on conditions such as pad rotation rate and polishing pressure. By adding potassium hydroxide (KOH) to the polishing solution, the MRR almost matches that obtained with conventional abrasive polishing, but achieves ultra-smooth surface finish (Ra: 0.388 nm). The polishing method presented here uses approximately 94% less Ce compared with conventional abrasive polishing, thereby dramatically reducing consumption of this valuable rare earth element.

Abrasive-free polishing of tungsten alloy using electrochemical etching

Tungsten alloy is a crucial engineering material for electrical and optical applications. However, damage-free and highly efficient polishing of tungsten has not been realized yet. We report the abrasive-free polishing of tungsten alloy using electrochemical polishing (ECP), which is an etching process. To achieve balance between polishing efficiency and surface quality, a two-step ECP process has been proposed. Current-driven ECP lasting for 3min, as the first step, quickly removed the surface grinding marks and subsurface damage while the following potential-driven ECP lasted for 20min, as the second step, improved the surface roughness and an ultra-smooth surface with an Ra roughness of 17.6nm was finally obtained.

Micro water dissolution machining principle and its application in ultra-precision processing of KDP optical crystal

In this paper, a micro water dissolution machining (MWDM) principle is proposed for machining potassium dihydrogen phosphate (KDP) crystal using water-in-oil micro-emulsion as an abrasive-free polishing fluid. In addition, two instances of the application of this principle to ultra-precision machining of KDP crystals are presented. Computer-controlled optical surfacing (CCOS) and diamond wire cutting (DWC) process were carried out according to the MWDM principle. In the case of the CCOS technology, it is found that the micro-waviness was removed completely by following the MWDM principle. The surface undulation decreased from 40 nm to less than 10 nm, and the surface root-mean-square (rms) roughness obviously reduced from 8.147 to 2.660 nm. In the case of the DWC process, the surface rms roughness reduced from 8.012 to 2.391 μm, and the cutting efficiency was improved. These results indicate that the MWDM principle can efficiently improve the machining quality of KDP optical crystal and has a great potential to machine water-soluble materials.

AIBA as Free Radical Initiator for Abrasive-Free Polishing of Hard Disk Substrate

In order to optimize the existing slurry for abrasive-free polishing (AFP) of a hard disk substrate, a water-soluble free radical initiator, 2,2′-azobis (2-methylpropionamidine) dihydrochloride (AIBA) was introduced into H2O2-based slurry in the present work. Polishing experiment results with AIBA in the H2O2 slurry indicate that the material removal rate (MRR) increases and the polished surface has a lower surface roughness. The mechanism of AIBA in AFP was investigated using electron spin-resonance spectroscopy and UV–Visible analysis, which showed that the concentration of hydroxyl radical (a stronger oxidizer than H2O2) in the slurry was enhanced in the present of AIBA. The structure of the film formed on the substrate surface was investigated by scanning electron microscopy, auger electron spectroscopy and electrochemical impedance spectroscopy technology, showing that a looser and porous oxide film was found on the hard disk substrate surface when treated with the H2O2-AIBA slurry. Furthermore, potentiodynamic polarization tests show that the H2O2-AIBA slurry has a higher corrosion current density, implying that a fast dissolution reaction can occur on the substrate surface. Therefore, we can conclude that the stronger oxidation ability, loose oxide film on the substrate surface, and the higher corrosion-wear rate of the H2O2-AIBA slurry lead to the higher MRR.

Abrasive-Free Polishing of Single-Crystal 4H-SiC with Silica Glass Plates

In this study, abrasive-free polishing of a single-crystal 4H-SiC (0001) substrate was investigated. Two different types of polishing plates, namely a synthetic SiO2 glass (quartz) plate and a soda-lime SiO2 glass plate, were used to polish the SiC substrate under atmospheric conditions at room temperature. The surface roughness after polishing was evaluated by differential interference contrast microscopy, phase-shift interferometric microscopy, and atomic force microscopy. In addition, the chemical bonding states of the SiC surface before and after polishing were analyzed by X-ray photoelectron spectroscopy. The experimental results showed that an oxide layer was formed on the SiC surface as a result of the chemical reaction between the interfaces of the synthetic SiO2 glass plate and the SiC substrate. This generated oxide layer was effectively removed by polishing with the soda-lime SiO2 glass plate, resulting in an atomically smooth SiC surface with a root mean square roughness of less than 0.1 nm for 1.5 h. Obtained experimental results indicate that the component materials, temperature and water adsorptive property of the soda-lime SiO2 glass play an important role in the removal of the tribochemically generated layer on the SiC surface during this polishing.

Dynamic Friction Polishing of Diamond Utilizing High Reactive Metallic Tools

The authors developed “Dynamic Friction Polishing (DF polishing) Method” utilizing a thermochemical reaction between a diamond workpiece and a metal. This method enables high efficiency abrasive-free polishing of single crystal and polycrystalline diamonds (PCD) by simply pressing them against a stainless steel (SUS304) disc rotating at a high peripheral speed (VS>2500m/min). In the authors’ previous paper, a top of the diamond test piece (0.6mm×0.6mm, (100) plane) was removed at a rate of 2.6mm/min (0.94mm3/min) under the polishing condition of sliding speed VS=4000m/min, loading pressure P=130MPa and polishing time t=10s. A bottleneck for practical use of this method is a high pressure over 100MPa required for pressing a diamond workpiece against a rotating stainless steel disc. In this paper, a high efficiency tool was manufactured by electro-spark deposition of highly reactive special metals on a base disc tool. Among various reactive metals Nb and W brought very high efficiency in the polishing of a single crystal diamond.

Chemical Mechanical Polishing of III-V Materials for Wafer Bonding Applications

ABSTRACT Wafer bonding of III-V cells offers opportunities to integrate materials with different bandgaps and different lattice parameters without forming high defect densities. However, growth conditions that produce the best cells do not necessarily produce epitaxial layer surfaces that are suitable for wafer bonding. The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and alloy layers used in high efficiency III-V solar cell structures is investigated with sodium hypochlorite and citric acid solutions. It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min. The low removal rate is key for applications that include epitaxial layers that must be subsequently polished without l=excessive loss of material. The actual composition of the slurry is adjusted for the particular alloy composition. A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid similar to Br-methanol polishing. However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams. Subsequent bonding suing sulfur passivation on the CMP surface shows excellent electrical characteristics with low resistivity as well as high strength bonding after annealing at 400 °C. Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, non-destructive method for evaluating damage induced by the CMP process.

Effect of Mn (II) Ion on Abrasive-Free Polishing of Hard Disk Substrate

With high requirement setting for hard disk substrate surface quality, abrasive-free polishing (AFP) has attracted more researchers’ attention. In this paper, the influence of Mn (Ⅱ) ion on AFP of hard disk substrate in the H2O2based slurry was investigated. The experiments results show that Mn (Ⅱ) ion can effectively increase the material remove rate (MRR) and improve the planarization of hard disk substrate. Furthermore, the acting mechanism of Mn (Ⅱ) ion in AFP of hard disk substrate was analyzed. The electron spin-resonance spectroscopy (EPR) analysis shows that Mn (Ⅱ) ion in the H2O2based slurry not only can increase the concentration of ·OH free radical, but also can make H2O2decompose to ·O2–free radical. These free radicals can accelerate the chemical etching and increase the MRR of hard disk substrate.

Abrasive-Free Chemical Mechanical Polishing of Hard Disk Substrate with Cumene Hydroperoxide-H2O2 Slurry

With magnetic heads operating closer to hard disks, the hard disks must be ultra-smooth. The abrasive-free polishing (AFP) performance of cumene hydroperoxide (CHP) as the initiator in H2O2-based slurry for hard disk substrate was investigated in our work, and the results showed that the slurry including CHP could improve the material removal rate (MRR) and also reduce surface roughness. Electron spin-resonance spectroscopy (EPR), electrochemical measurement and Auger electron spectroscopy (AES) were conducted to investigate the acting mechanism with CHP during the polishing process. Compared with the H2O2 slurry, the EPR analysis shows that the CHP–H2O2 slurry provides a higher concentration of the HOO free radical. In addition, the AES analysis shows the oxidization reaction occurs in the external layer of the substrate surface. Furthermore, electrochemical measurements reveal that CHP can promote the electrochemical effect in AFP and lead to the increase of MRR.

Effect of AIBA on Abrasive-Free Polishing of Hard Disk Substrate with Peroxyacetic Acid System Slurry

In the present study, the effect of 2, 2`-Azobis (2-methylpropionamidine) dihydrochloride (AIBA) on the abrasive-free polishing (AFP) of hard disk substrate was investigated. Polishing experiment results indicate that the material removal rate (MRR) of hard disk substrate polished in slurry containing AIBA and peroxyacetic acid (PAA) is obviously higher than that without AIBA. Furthermore, the acting mechanism of AIBA in AFP of hard disk substrate was analyzed. Auger electron spectrometer results demonstrate that the oxidation reaction occurred on the surface and formed a passivation layer. The potentiodynamic polarization tests show that AIBA can increase the corrosion rate of the hard disk substrate in PAA-based slurry, and accelerate the dissolution reaction of oxidation film, which lead to the improved MRR in AFP process.

Abrasive-Free Chemical Polishing of Hard Disk Substrate with Benzoyl Peroxide-H<sub>2</sub>O<sub>2</sub> Slurry

The effect of benzoyl peroxide (BPO) as an initiator in H2O2slurry for abrasive-free chemical polishing of hard disk substrate was investigated. The results of abrasive-free polishing tests show that the introduction of BPO increases material removal rate (MRR) and decreases the value of Roughness (Ra). To further investigate the mechanism of abrasive-free polishing, electron spin-resonance spectroscopy (EPR) tests, auger electron spectrometer (AES) and electrochemical tests were conducted. Electron spin-resonance spectroscopy (EPR) tests show the concentration of radicals increase and promote the reaction on the surface of hard disk substrate. Auger electron spectrometer (AES) and electrochemical tests indicate chemical changes happen on the surface of hard disk substrate, and the formed oxide film may be sparse or porous. The results imply that the introduction of BPO can effectively improve the effect of abrasive-free polishing.

Influence of Zn (II) ion on abrasive-free polishing of hard disk substrate

With higher requirement setting for hard disk substrate to minimize roughness and defects of the polished surface, abrasive-free polishing (AFP) of hard disk substrate has been put forward in this paper. The effect of Zn (II) ion on the AFP of hard disk substrate in the H2O2 based slurry was investigated by AFP tests. The results indicate that the material removal rate of hard disk substrate polished in slurry with Zn (II) ion is obviously higher than that without Zn (II) ion. And surface polished by slurry containing Zn (II) ion exhibits lower surface roughness and fewer nano-asperity peaks than that without Zn (II) ion. Furthermore, the acting mechanism of Zn (II) ion in AFP of hard disk substrate was analyzed. X-ray photoelectron spectroscopy analysis shows that metal Zn appears on the polished surface, implying the tribochemistry reaction occurs during AFP. The electrochemical reaction between metal Zn and oxide film Ni2O3 on the surface of hard disk substrate during AFP can promote the chemical effect in AFP and lead to the increasing of material removal rate.

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.