Direct adhesion between Cu foil and polytetrafluoroethylene without increasing surface roughness for high-frequency printed wiring boards.

Polytetrafluoroethylene (PTFE) serves as a suitable dielectric substrate for high-frequency printed wiring boards (PWBs) owing to its excellent properties at high frequency. However, to the best of our knowledge, no study has investigated the strong adhesion between PTFE and Cu foil with low surface roughness. Therefore, in this study, pure-PTFE comprising a weak boundary layer (WBL) on the surface and glass-cloth-containing PTFE (GC-PTFE), which did not contain a WBL, were subjected to heat-assisted plasma (HAP) treatment. Thereafter, we investigated the surface chemical bonding state, surface morphology, and adhesion properties of the as-prepared PTFE toward Cu foil with low surface roughness. As observed, oxygen-containing functional groups were generated on the HAP-treated PTFE, and the WBL in the as-received pure-PTFE was eliminated via HAP treatment. Moreover, the surface roughness of the HAP-treated PTFE did not increase compared to that of as-received PTFE. After performing thermal compression under atmospheric conditions, the adhesion strength of both HAP-treated pure-PTFE and GC-PTFE was ∼0.9 N mm-1. In addition, the adhesion strength of Cu/pure-PTFE and Cu/GC-PTFE increased after thermal compression under a reduced pressure, and the adhesion strength of 1 N mm-1 was obtained. Although the Cu foil was not roughened, Cu/PTFE realized strong adhesive strength. The developed method is advantageous because maintaining a low interface roughness is crucial for applying PTFE to manufacture high-frequency PWBs.

Surface modification and adhesive-free adhesion of polytetrafluoroethylene (PTFE) and silicone gel containing oleophilic SiO2 powder by plasma treatment.

Polytetrafluoroethylene (PTFE) has high-frequency characteristics and low transmission loss, and is expected to be used as a substrate material of printed wiring board for high-frequency applications. Meanwhile, silicone gel has superior properties such as attaching/detaching, weather resistance, and human safety. If the PTFE and silicone gel can be strongly adhered to, they can be applied to internet of things (IoT) devices that can be attached and detached freely. However, adhesion between PTFE, which has poor adhesion, and silicone gel, which has low mechanical strength, is difficult and has not been reported. In this study, PTFE was modified with heat-assisted plasma treatment, and silicone gel was treated with oleophilic SiO2 powder to improve elastic modulus and modified with plasma jet treatment, and then bonded without adhesive. The adhesion strength of PTFE/silicone gel assembly was 1.13 N mm-1 when treated moderately, but only 0.01 N mm-1 when untreated and treated excessively. To investigate the factors causing the difference in the adhesion strength, the surface of silicone gel was evaluated by water contact angle measurement, Fourier transform infrared spectroscopy, and confocal laser scanning microscopy. When treated moderately, hydrophilic functional groups and cross-linking were most frequently increased. Furthermore, when treated excessively, surface degradation was observed, which was expected to lower the adhesion strength. The adhesive-free bonding between PTFE and silicone gel can open a new path for developing IoT devices that can be freely attached and detached.

Flexible selection of the functional-group ratio on a polytetrafluoroethylene (PTFE) surface using a single-gas plasma treatment.

During plasma treatment of polymers, etching occurs and functional groups are introduced on their surface. We assumed that controlling the etching rate would enable plasma treatment using a single gas to control the ratio of functional groups generated on a polymer's surface, although previous studies have indicated that several different types of functional groups are formed when the gaseous species are varied. In this study, we selected the base pressure (BP) as a parameter for controlling the etching rate and subjected polytetrafluoroethylene (PTFE) to plasma treatments using only He gas at various BPs. The chemical composition of the surface of the plasma-treated PTFE samples was evaluated by X-ray photoelectron spectroscopy (XPS), and the ratios of fluorine (CF3, CF2, C-F), oxygen (O-C[double bond, length as m-dash]O, C[double bond, length as m-dash]O, C-O), and carbon (C-C, C[double bond, length as m-dash]C) groups were quantified from the C 1s-XPS spectra. The fluorine-group ratio decreased and the oxygen- and carbon-group ratios increased with decreasing BP. The results demonstrated that plasma treatment using a single gas enabled flexible selection of the ratio of functional groups generated on PTFE via control of the BP.

Separation of Neighboring Terraces on a Flattened Si(111) Surface by Selective Etching along Step Edges Using Total Wet Chemical Processing.

We propose a bottom-up technique using total wet chemical treatments to separate neighboring terraces on Si(111). First, Ag cations were reduced at the step edges of a vicinal Si(111) surface composed of biatomic steps and flat terraces, resulting in self-assembled Ag rows consisting of nanodots and nanowires. By immersing this sample into a mixed solution of HF and H2O2, almost continuous nanotrenches with depths and widths of nanometer scales were fabricated along the edges. The potential electrochemical processes in the solution/Ag/Si system that lead to the formation of nanotrenches are discussed. Additionally, we present how we plan to use our approach to create atomic-thickness Si ribbons.

Adhesive-Free Adhesion between Plasma-Treated Glass-Cloth-Containing Polytetrafluoroethylene (GC-PTFE) and Stainless Steel: Comparison between GC-PTFE and Pure PTFE.

In this study, the effect of plasma treatment on glass-cloth-containing polytetrafluoroethylene (GC-PTFE) was investigated. Previous plasma studies investigated pure PTFE (which does not contain glass cloth) but not GC-PTFE. The effect of Ar + H2O plasma treatment on GC-PTFE was investigated. The Ar + H2O plasma-treated GC-PTFE sheets were thermally compressed to stainless steel (SUS304) foils without using adhesive, and the GC-PTFE/SUS304 adhesion strengths were measured using a 90° peel test. The adhesion strength increased with the increase in the plasma treatment time (0.8 and 1.0 N/mm at 20 s and 300 s, respectively). Thus, strong adhesion between GC-PTFE/SUS304 was achieved without adhesive. This improvement in the adhesion properties of GC-PTFE can be attributed to the generation of oxygen-containing functional groups and the decrease in the surface roughness of the samples. Thereafter, the adhesion properties of GC-PTFE and pure PTFE were compared. Because, unlike pure PTFE, GC-PTFE has no weak boundary layer, GC-PTFE exhibited better adhesion properties than pure PTFE under short plasma treatment times.

Effects of He and Ar Heat-Assisted Plasma Treatments on the Adhesion Properties of Polytetrafluoroethylene (PTFE).

Heat-assisted plasma (HAP) treatment using He gas is known to improve the adhesive-bonding and adhesive-free adhesion properties of polytetrafluoroethylene (PTFE). In this study, we investigated the effects of He and Ar gaseous species on the HAP-treated PTFE surface. Epoxy (EP) adhesive-coated stainless steel (SUS304) and isobutylene-isoprene rubber (IIR) were used as adherents for the evaluation of the adhesive-bonding and adhesive-free adhesion properties of PTFE. In the case of adhesive bonding, the PTFE/EP-adhesive/SUS304 adhesion strength of the Ar-HAP-treated PTFE was the same as that of the He-HAP-treated PTFE. In the case of adhesive-free adhesion, the PTFE/IIR adhesion strength of the Ar-HAP-treated PTFE was seven times lower than that of the He-HAP-treated PTFE. The relation among gaseous species used in HAP treatment, adhesion properties, peroxy radical density ratio, surface chemical composition, surface modification depth, surface morphology, surface hardness, and the effect of irradiation with vacuum ultraviolet (VUV) and UV photons were investigated. The different adhesive-free adhesion properties obtained by the two treatments resulted from the changes in surface chemical composition, especially the ratios of oxygen-containing functional groups and C-C crosslinks.

Damage-free highly efficient plasma-assisted polishing of a 20-mm square large mosaic single crystal diamond substrate.

Plasma-assisted polishing (PAP) as a damage-free and highly efficient polishing technique has been widely applied to difficult-to-machine wide-gap semiconductor materials such as 4H-SiC (0001) and GaN (0001). In this study, a 20-mm square large mosaic single crystal diamond (SCD) substrate synthesized by microwave plasma chemical vapor deposition (CVD) was polished by PAP. Argon-based plasma containing oxygen was used in PAP to modify the surface of quartz glass polishing plate, and a high material removal rate (MRR) of 13.3 µm/h was obtained. The flatness of SCD polished by PAP measured by an interferometer was 0.5 µm. The surface roughness measured by both scanning white light interferometer (SWLI) (84-µm square) and atomic force microscope (AFM) (5-µm square) was less than 0.5 nm Sq. The micro-Raman spectroscopy measurement results of mosaic SCD substrate processed by PAP showed that residual stress and non-diamond components on the surface after PAP processing were below the detection limit.

Strong Biomimetic Immobilization of Pt-Particle Catalyst on ABS Substrate Using Polydopamine and Its Application for Contact-Lens Cleaning with H2O2.

Polydopamine (PDA)-a known adhesive coating material-was used herein to strongly immobilize a Pt-particle catalyst on an acrylonitrile-butadiene-styrene copolymer (ABS) substrate. Previous studies have shown that the poor adhesion between Pt particles and ABS surfaces is a considerable problem, leading to low catalytic durability for H2O2 decomposition during contact-lens cleaning. First, the ABS substrate was coated with PDA, and the PDA film was evaluated by X-ray photoelectron spectroscopy. Second, Pt particles were immobilized on the PDA-coated ABS substrate (ABS-PDA) using the electron-beam irradiation reduction method. The Pt particles immobilized on ABS-PDA (Pt/ABS-PDA) were observed using a scanning electron microscope. The Pt-loading weight was measured by inductively coupled plasma atomic emission spectroscopy. Third, the catalytic activity of the Pt/ABS-PDA was evaluated as the residual H2O2 concentration after immersing it in a 35,000-ppm H2O2 solution (the target value was less than 100 ppm). The catalytic durability was evaluated as the residual H2O2 concentration after repeated use. The PDA coating drastically improved both the catalytic activity and durability because of the high Pt-loading weight and strong adhesion among Pt particles, PDA, and the ABS substrate. Plasma treatment prior to PDA coating further improved the catalytic durability.

Improved Catalytic Durability of Pt-Particle/ABS for H2O2 Decomposition in Contact Lens Cleaning.

In a previous study, Pt nanoparticles were supported on a substrate of acrylonitrile⁻butadiene⁻styrene copolymer (ABS) to give the ABS surface catalytic activity for H2O2 decomposition during contact lens cleaning. Although the Pt-particle/ABS catalysts exhibited considerably high specific catalytic activity for H2O2 decomposition, the catalytic activity decreased with increasing numbers of repeated usage, which meant the durability of the catalytic activity was low. Therefore, to improve the catalytic durability in this study, we proposed two types of pretreatments, as well as a combination of these treatments before supporting Pt nanoparticles on the ABS substrate. In the first method, the ABS substrate was etched, and in the second method, the surface charge of the ABS substrate was controlled. A combination of etching and surface charge control was also applied as a third method. The effects of these pretreatments on the surface morphology, surface chemical composition, deposition behavior of Pt particles, and Pt loading weight were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cross-sectional SEM, and inductively coupled plasma atomic emission spectroscopy (ICP-AES), respectively. Both etching and controlling the surface charge effectively improved the catalytic durability for H2O2 decomposition. In addition, the combination treatment was the most effective.

Influence of air contamination during heat-assisted plasma treatment on adhesion properties of polytetrafluoroethylene (PTFE).

Plasma surface treatment is typically not effective on fluoropolymers containing polytetrafluoroethylene (PTFE). It is reported that heat-assisted plasma (HAP) treatment at high temperatures (above 200 °C) under atmospheric pressure helium (He) plasma improves the adhesion properties of PTFE. In this study, we investigated the influence of the air concentration during HAP treatment on the adhesion properties of PTFE. Air concentration was controlled via ambient air inflow amount, in other words, base pressure. The PTFE samples HAP-treated in different air concentrations were thermally compressed with an unvulcanized isobutylene-isoprene rubber (IIR). Then, the PTFE/IIR adhesion strength was measured via T-peel test. We show that, when PTFE was HAP-treated in 0.01% air, its PTFE/IIR adhesion strength was over 2 N mm-1; the IIR underwent cohesion failure. However, the PTFE/IIR adhesion strength drastically decreased in the presence of air contamination. The relationships between air concentration during HAP treatment, adhesion properties of PTFE, surface chemical composition, surface morphology, and surface hardness were investigated and discussed.

Surface Modification and Microstructuring of 4H-SiC(0001) by Anodic Oxidation with Sodium Chloride Aqueous Solution.

Anodic oxidation is a promising surface modification technique for the manufacture of SiC wafers owing to its high oxidation rate. It is also possible to fabricate porous SiC by anodic oxidation and etching owing to the material properties of SiC. In this study, the anodic oxidation of a 4H-SiC(0001) surface was investigated by performing repeated anodic oxidation and hydrofluoric acid etching on a 4H-SiC(0001) surface, during which the formation of porous SiC was observed and studied. Anodic oxidation is very effective for removing the surface damage formed by mechanical polishing, and the surface after removing the surface damage can be oxidized uniformly and has a higher oxidation rate than a surface newly finished by chemical mechanical polishing (CMP). We proposed a model based on the electrochemical impedance method to explain the difference in the oxidation between an as-CMP-finished surface and an oxidized/etched surface. Porous SiC was obtained in this study, which was due to the anisotropy of the SiC crystal. The structure of the porous SiC was significantly dependent on the etch pits generated at the beginning of anodic oxidation and can be controlled via anodic oxidation parameters. Anodic oxidation and hydrofluoric acid etching cannot remove porous SiC owing to the anisotropic oxidation of the SiC surface and the difficulty of anodizing SiC fibers. This study shows that anodic oxidation is a promising technique for the modification of SiC surfaces and the fabrication of porous SiC.

Adhesive-free adhesion between heat-assisted plasma-treated fluoropolymers (PTFE, PFA) and plasma-jet-treated polydimethylsiloxane (PDMS) and its application.

Conventional low-temperature plasma treatment was reported to minimally improve the adhesion property of polytetrafluoroethylene (PTFE), whereas heat-assisted plasma (HAP) treatment significantly improved the same. An unvulcanized rubber was previously used as an adherent for PTFE. This study aimed to achieve strong adhesive-free adhesion between PTFE and vulcanized polydimethylsiloxane (PDMS) rubber. As-received vulcanized PDMS rubber did not adhere to HAP-treated PTFE, and as-received PTFE did not adhere to vulcanized rubber of plasma-jet (PJ) treated PDMS rubber; however, HAP-treated PTFE strongly adhered to vulcanized PJ-treated PDMS rubber, and both PTFE and PDMS exhibited cohesion failure in the T-peel test. The surface chemical compositions of the PTFE and PDMS sides were determined using X-ray photoelectron spectroscopy. The strong PTFE/PDMS adhesion was explained via hydrogen and covalent bond formation (C-O-Si and/or C(=O)-O-Si) between hydroxyl (C-OH) or carboxyl (C(=O)-OH) groups of the HAP-treated PTFE. This process was also applied to adhesive-free adhesion between a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and PDMS; subsequently, a translucent PFA/PDMS assembly with strong adhesion was realized together with the PTFE/PDMS assembly. Strong adhesive-free adhesion between fluoropolymers (PTFE, PFA) and vulcanized PDMS rubber without using any adhesives and graft polymer was successfully realized upon plasma treatment of both the fluoropolymer and PDMS sides. Additionally, a PDMS sheet, which was PJ-treated on both sides, was applied to strongly adhere fluoropolymers (PTFE, PFA) to materials such as metal and glass. PJ-treated PDMS was used as an intermediate layer rather than a strong adhesive, achieving PTFE/PDMS/metal and PTFE/PDMS/glass assemblies. The PTFE/PDMS, PDMS/metal, and PDMS/glass adhesion strengths exceeded 2 N/mm.

Optimization of Gas Composition Used in Plasma Chemical Vaporization Machining for Figuring of Reaction-Sintered Silicon Carbide with Low Surface Roughness.

In recent years, reaction-sintered silicon carbide (RS-SiC) has been of interest in many engineering fields because of its excellent properties, such as its light weight, high rigidity, high heat conductance and low coefficient of thermal expansion. However, RS-SiC is difficult to machine owing to its high hardness and chemical inertness and because it contains multiple components. To overcome the problem of the poor machinability of RS-SiC in conventional machining, the application of atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) to RS-SiC was proposed. As a highly efficient and damage-free figuring technique, AP-PCVM has been widely applied for the figuring of single-component materials, such as Si, SiC, quartz crystal wafers, and so forth. However, it has not been applied to RS-SiC since it is composed of multiple components. In this study, we investigated the AP-PCVM etching characteristics for RS-SiC by optimizing the gas composition. It was found that the different etching rates of the different components led to a large surface roughness. A smooth surface was obtained by applying the optimum gas composition, for which the etching rate of the Si component was equal to that of the SiC component.

Radiolytic Synthesis of Pt-Particle/ABS Catalysts for H2O2 Decomposition in Contact Lens Cleaning.

A container used in contact lens cleaning requires a Pt plating weight of 1.5 mg for H2O2 decomposition although Pt is an expensive material. Techniques that decrease the amount of Pt are therefore needed. In this study, Pt nanoparticles instead of Pt plating film were supported on a substrate of acrylonitrile-butadiene-styrene copolymer (ABS). This was achieved by the reduction of Pt ions in an aqueous solution containing the ABS substrate using high-energy electron-beam irradiation. Pt nanoparticles supported on the ABS substrate (Pt-particle/ABS) had a size of 4-10 nm. The amount of Pt required for Pt-particle/ABS was 250 times less than that required for an ABS substrate covered with Pt plating film (Pt-film/ABS). The catalytic activity for H2O2 decomposition was estimated by measuring the residual H2O2 concentration after immersing the catalyst for 360 min. The Pt-particle/ABS catalyst had a considerably higher specific catalytic activity for H2O2 decomposition than the Pt-film/ABS catalyst. In addition, sterilization performance was estimated from the initial rate of H2O2 decomposition over 60 min. The Pt-particle/ABS catalyst demonstrated a better sterilization performance than the Pt-film/ABS catalyst. The difference between Pt-particle/ABS and Pt-film/ABS was shown to reflect the size of the O2 bubbles formed during H2O2 decomposition.

Drastic Improvement in Adhesion Property of Polytetrafluoroethylene (PTFE) via Heat-Assisted Plasma Treatment Using a Heater.

The heating effect on the adhesion property of plasma-treated polytetrafluoroethylene (PTFE) was examined. For this purpose, a PTFE sheet was plasma-treated at atmospheric pressure while heating using a halogen heater. When plasma-treated at 8.3 W/cm2 without using the heater (Low-P), the surface temperature of Low-P was about 95 °C. In contrast, when plasma-treated at 8.3 W/cm2 while using the heater (Low-P+Heater), the surface temperature of Low-P+Heater was controlled to about 260 °C. Thermal compression of the plasma-treated PTFE with or without heating and isobutylene-isoprene rubber (IIR) was performed, and the adhesion strength of the IIR/PTFE assembly was measured via the T-peel test. The adhesion strengths of Low-P and Low-P+Heater were 0.12 and 2.3 N/mm, respectively. Cohesion failure of IIR occurred during the T-peel test because of its extremely high adhesion property. The surfaces of the plasma-treated PTFE with or without heating were investigated by the measurements of electron spin resonance, X-ray photoelectron spectroscopy, nanoindentation, scanning electron microscopy, and scanning probe microscopy. These results indicated that heating during plasma treatment promotes the etching of the weak boundary layer (WBL) of PTFE, resulting in a sharp increase in the adhesion property of PTFE.

Competition between surface modification and abrasive polishing: a method of controlling the surface atomic structure of 4H-SiC (0001).

The surface atomic step-terrace structure of 4H-SiC greatly affects its performance in power device applications. On the basis of the crystal structure of 4H-SiC, we propose the generation mechanism of the a-b-a*-b* type, a-b type and a-a type step-terrace structures. We demonstrate that the step-terrace structure of SiC can be controlled by adjusting the balance between chemical modification and physical removal in CeO2 slurry polishing. When chemical modification plays the main role in the polishing of SiC, the a-b-a*-b* type step-terrace structure can be generated. When the roles of physical removal and chemical modification have similar importance, the a-b-a*-b* type step-terrace structure changes to the a-b type. When physical removal is dominant, the uniform a-a type step-terrace structure can be generated.

Comparative analysis of oxidation methods of reaction-sintered silicon carbide for optimization of oxidation-assisted polishing.

Combination of the oxidation of reaction-sintered silicon carbide (RS-SiC) and the polishing of the oxide is an effective way of machining RS-SiC. In this study, anodic oxidation, thermal oxidation, and plasma oxidation were respectively conducted to obtain oxides on RS-SiC surfaces. By performing scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX) analysis and scanning white light interferometry (SWLI) measurement, the oxidation behavior of these oxidation methods was compared. Through ceria slurry polishing, the polishing properties of the oxides were evaluated. Analysis of the oxygen element on polished surfaces by SEM-EDX was conducted to evaluate the remaining oxide. By analyzing the three oxidation methods with corresponding polishing process on the basis of schematic diagrams, suitable application conditions for these methods were clarified. Anodic oxidation with simultaneous polishing is suitable for the rapid figuring of RS-SiC with a high material removal rate; polishing of a thermally oxidized surface is suitable for machining RS-SiC mirrors with complex shapes; combination of plasma oxidation and polishing is suitable for the fine finishing of RS-SiC with excellent surface roughness. These oxidation methods are expected to improve the machining of RS-SiC substrates and promote the application of RS-SiC products in the fields of optics, molds, and ceramics.

Ultrasmooth reaction-sintered silicon carbide surface resulting from combination of thermal oxidation and ceria slurry polishing.

An ultrasmooth reaction-sintered silicon carbide surface with an rms roughness of 0.424 nm is obtained after thermal oxidation for 30 min followed by ceria slurry polishing for 30 min. By SEM-EDX analysis, we investigated the thermal oxidation behavior of RS-SiC, in which the main components are Si and SiC. As the oxidation rate is higher in the area with defects, there are no scratches or cracks on the surface after oxidation. However, a bumpy structure is formed after oxidation because the oxidation rates of Si and SiC differ. Through a theoretical analysis of thermal oxidation using the Deal-Grove model and the removal of the oxide layer by ceria slurry polishing in accordance with the Preston equation, a model for obtaining an ultrasmooth surface is proposed and the optimal processing conditions are presented.

Fundamental research on the label-free detection of protein adsorption using near-infrared light-responsive plasmonic metal nanoshell arrays with controlled nanogap.

In this work, we focused on the label-free detection of simple protein binding using near-infrared light-responsive plasmonic nanoshell arrays with a controlled interparticle distance. The nanoshell arrays were fabricated by a combination of colloidal self-assembly and subsequent isotropic helium plasma etching under atmospheric pressure. The diameter, interparticle distance, and shape of nanoshells can be tuned with nanometric accuracy by changing the experimental conditions. The Au, Ag, and Cu nanoshell arrays, having a 240-nm diameter (inner, 200-nm polystyrene (PS) core; outer, 20-nm metal shell) and an 80-nm gap distance, exhibited a well-defined localized surface plasmon resonance (LSPR) peak at the near-infrared region. PS@Au nanoshell arrays showed a 55-nm red shift of the maximum LSPR wavelength of 885 nm after being exposed to a solution of bovine serum albumin (BSA) proteins for 18 h. On the other hand, in the case of Cu nanoshell arrays before/after incubation to the BSA solution, we found a 30-nm peak shifting. We could evaluate the difference in LSPR sensing performance by changing the metal materials.

Shape correction of optical surfaces using plasma chemical vaporization machining with a hemispherical tip electrode.

We propose a plasma chemical vaporization machining device with a hemispherical tip electrode for optical fabrication. Radio-frequency plasma is generated close to the electrode under atmospheric conditions, and a workpiece is scanned relative to the stationary electrode under three-axis motion control to remove target areas on a workpiece surface. Experimental results demonstrate that surface removal progresses although process gas is not forcibly supplied to the plasma. The correction of shape errors on conventionally polished spheres is performed. As a result, highly accurate smooth surfaces with the desired rms shape accuracy of 3 nm are successfully obtained, which confirms that the device is effective for the fabrication of optics.