Pressure Plasma Processing Research Articles

Combined processing chain for freeform optics based on atmospheric pressure plasma processing and bonnet polishing.

Development of major optical projects demands great amounts of freeform optics, which requires mass-production technology. The atmospheric pressure plasma processing (APPP) based on chemical etching shows the advantage of high efficiency in form generation. However, the surface texture gets deteriorated with roughness to 60 nm Ra. Bonnet polishing can achieve ultra-smooth surface texture using flexible inflated tool. In this study, a combined processing chain consisting of APPP and bonnet polishing was proposed, in which APPP plays a role in form generation and bonnet polishing plays a role in rapid surface smoothing. Firstly, the form generation using APPP and the surface finishing using bonnet polishing were analyzed. Then the form-preserving ability of bonnet polishing under various conditions, including polishing parameters and surface form characteristics, was investigated experimentally. Consequently, the influence of bonnet polishing on surface form change was analyzed. Two sinusoidal freeform surfaces with 5 mm and 16 mm spatial period were generated using APPP and then smoothed using bonnet polishing. The experimental results show that the polishing induced form change is smaller than 20 nm PV with roughness improved to 2 nm Ra, which verifies the feasibility of the combined processing chain.

Atmospheric-pressure plasma oxidation of aluminum for large-area electronics

We developed an atmospheric pressure plasma (APP) process for forming metal oxides and self-assembled monolayers (SAMs) without a vacuum process. SAMs are frequently utilized for surface modification. In particular, a combination of aluminum oxide (AlOx) and alkylphosphonic acid is used as the gate dielectric in flexible electronics, with a low operating voltage of 2 V because of its simple fabrication process at room temperature in air. However, when using a combination of SAMs and AlOx as the gate dielectric, it is necessary to form a metal oxide with sufficient thickness using a vacuum plasma process to reduce the leakage current, which is not suitable for use in flexible large-area electronics. In this study, we resolved this problem by utilizing an APP process and substrate heating instead of the vacuum plasma process. Heating a substrate above 100 °C forms a sufficient AlOx layer. The leakage current of plasma-processed AlOx with SAMs was reduced by three orders of magnitude compared to that of natural AlOx with SAMs. X-ray photoelectron spectroscopy revealed a time dependence of the thickness of AlOx. An exposure time of 100 s was sufficient to form an AlOx layer with a thickness of 4 nm. An organic transistor with this APP treatment of AlOx and SAMs showed a mobility of 1.4 cm2/V s and an on/off ratio of 104 within 2 V. We also fabricated an inverter and a ring oscillator circuit with an inverter gain of more than 300 and an oscillation frequency of 40 Hz.

Characterization of fused silica surface topography in capacitively coupled atmospheric pressure plasma processing

Abstract Based on the pure chemical etching, atmospheric plasma processing techniques have been developed for fused silica optics fabrication, in order to achieve deterministic high rate material removal, small tool spot and no mechanical load applied. However, the surface smoothness tends to be deteriorated after etching process. Comprehensive characterization of surface topography after atmospheric plasma processing is necessary in order to understand the opacification phenomenon and etching mechanism. In this paper, a capacitively coupled atmospheric pressure plasma processing (CCAPPP) system and experimental setup are firstly presented. Chemical composition, surface topography, cross-section topography as well as quantitative surface roughness are respectively characterized and analyzed in detail. The results show that the topography difference between the transparent and the opaque area was not caused by chemical composition. The main differences in the microscopic topography were the size and density of etched cellular microstructures. The opacification phenomenon mainly resulted from the excessive roughening on the processed surface topography, causing visible light to be diffusely reflected.

Effect of Plasma Treatment of Titanium Surface on Biocompatibility

It was recently reported that implant osseointegration is affected by surface wettability. The relationship between hydrophilicity and cell adhesion was corroborated by numerous in vivo studies. Concentrated alkali improves the biocompatibility of pure titanium. Research was conducted on the mechanism by which this treatment increases hydrophilicity. In the present study, we used atmospheric pressure plasma processing to enhance the hydrophilicity of the material surface. The aim was to assess its influences on the initial adhesion of the material to rat bone marrow and subsequent differentiation into hard tissue. Superhydrophilicity was induced on a pure titanium surface with a piezobrush, a simple, compact alternative to the conventional atmospheric pressure plasma device. No structural change was confirmed by Scanning electron microscope (SEM) or scanning probe microscopy (SPM) observation. X-ray photoelectron spectroscopy (XPS) analysis presented with hydroxide formation and a reduction in the C peak. A decrease in contact angle was also observed. The treated samples had higher values for in vitro bovine serum albumin (BSA) adsorption, rat bone marrow (RBM) cell initial adhesion, alkaline phosphatase activity (ALP) activity tests, and factors related to bone differentiation than the untreated control. The present study indicated that the induction of superhydrophilicity in titanium via atmospheric pressure plasma treatment with a piezobrush affects RBM cell adhesion and bone differentiation without altering surface properties.

Continuous Phase Plate Structuring by Multi-Aperture Atmospheric Pressure Plasma Processing.

A multi-aperture atmospheric pressure plasma processing (APPP) method was proposed to structure the continuous phase plate (CPP). The APPP system was presented and removal investigation showed the removal function of APPP was of a high repeatability and robustness to the small disturbance of gas flows. A mathematical model for the multi-aperture structuring process was established and the simulation analysis indicated the advantages of the proposed method in terms of balancing the efficiency and accuracy of the process. The experimental results showed that multi-aperture APPP has the ability to structure a 30 mm × 30 mm CPP with the accuracy of 163.4 nm peak to valley (PV) and 31.7 nm root mean square (RMS).

Modelling of removal characteristics and surface morphology formation in capacitively coupled atmospheric pressure plasma processing of fused silica optics

Capacitively coupled atmospheric pressure plasma processing (CCAPPP) has been developed as a sub-aperture figuring tool for high precision fused silica optics, due to deterministic high rate material removal, small tool spot and no induced subsurface damage. In order to carry out an in-depth understanding on the removal and surface morphology formation mechanism of CCAPPP, this study aims to model the plasma discharge process and surface chemical reaction using the multi-physics simulation. The discharge characteristics such as electron density, electron temperature and particle density in the plasma are firstly obtained. Reaction gas components (CF4 and O2) are also added, and the main chemical reactions are analyzed by zero-dimensional modelling. Then the distribution of active atoms (active F atoms, O atoms and CFx molecules) related to the removal process is simulated in the full CCAPPP model. Finally, experiments are carried out to verify the simulation results, indicating that the distribution of active F atoms on the workpiece surface determines the Gaussian removal profile and the ratio of O/CFx is the key factor affecting the surface morphology formation of CCAPPP.

Fabrication of continuous phase plate using atmospheric pressure plasma processing

Continuous phase plate (CPP) with complex surface structures is used to smooth the focal spot of laser beam. Great demand for CPP in laser fusion system requires mass production technology with low cost. Therefore, the rapid fabrication method for CPP using atmospheric pressure plasma processing (APPP) is presented in this paper. Firstly, the fundamental characteristics of APPP were studied to obtain the optimal processing parameters. Considering the influence on etching rate from surface temperature, trench machining experiments were conducted to establish the exponential model of influence function. Based on this model, the iterative calculation method for dwell time was developed to minimize the thermal effect on machining error, and the sinusoidal surface experiments prove its effectiveness. At last, a 320 mm × 320 mm × 2 mm CPP of BOROFLOAT33 (B33) with 2.78 μm PV was fabricated within 16 h using APPP, and the RMS of form error is 96 nm. The far field focal spot of the machined CPP was calculated. The result shows that the machined CPP has an acceptable beam-shaping function, which demonstrates the potential for fabricating CPP using APPP.

Thermal analysis of inductively coupled atmospheric pressure plasma jet and its effect for optical processing

As an innovative optical processing technology based on the chemical reaction, inductively coupled atmospheric pressure plasma processing (IC-APPP) technology has the advantages of high processing efficiency, non-mechanical damage and low cost. Inductively coupled plasma jet (ICP) can be characterized by the high temperature and enthalpy, which is the prerequisite of high material removal rate in optical processing. Thermal analysis is critical to characterize the IC-APPP of optics. Based on the energy balance theory in the low-temperature plasma processing, the heat transfer process of substrate was analyzed and boundary conditions in the model were proposed. The gas temperature of the impinging jet was obtained by the simulation of plasma jet characteristics using finite element analysis, which acts as the input to the heat transfer model of IC-APPP process. Experiments were carried out to validate the accuracy of model and thermal effect on the removal rate was analyzed for IC-APPP. Results suggest that there exist varying degrees of nonlinearity with different temperature ranges in the IC-APPP.

Super-hydrophobic film deposition by an atmospheric-pressure plasma process and its anti-icing characteristics

In this work, the super-hydrophobic (SH) surface was prepared through chemical vapor deposition process by an argon atmospheric pressure plasma jet source with HMDSN (hexamethyldisilazane) as the polymerization precursor. Plasma synthesized organosilicon (SiOxCyHz) thin films with water contact angle over 160° and sliding angle below 5°, were able to be achieved. FTIR and XPS analysis indicates a large number of hydrocarbon compositions were polymerized in the thin films enduing the latter very-low surface free energy. SEM shows the SH films display micro-nanostructure and with high degree of averaged surface roughness 190 nm evaluated by AFM analysis. From experiments under controlled low-temperature and moisture conditions, the prepared SH surface exhibits good anti-icing effects. Significantly prolonging freezing time was achievable on the SH thin films for both static and sliding water droplets. This investigation demonstrates the anti-icing potentials of SH surface prepared through low-cost simple atmospheric-pressure plasma polymerization process.

Fine-tuning of chemical and physical polymer surface modifications by atmospheric pressure post-discharge plasma and its correlation with adhesion improvement

Our study focuses on the use of a remote atmospheric pressure plasma process for the improvement of polymer surface reactivity. Indeed, surface activation of carbon-fiber-reinforced polymers that use a polyetheretherketone matrix is required to achieve strong and long-term adhesion of paint on the composite. Emphasis will be placed on the contribution of gas plasma used on surface modification. We use two techniques to characterize adherence improvement: a qualitative and industrial approach with a cross-cut test and a quantitative approach with a three-point bending test. However, to provide a better understanding of the plasma-induced modifications using air or nitrogen gas, diagnostics of the surface are also necessary. The wettability improvement, chemical surface modifications, and topography are analyzed by using contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy, respectively. The combination of these diagnostics highlights the capability of plasma treatment to tune the plasma/surface interactions depending on gas plasma, which results in improved adhesion.

Thermal modelling for conformal polishing in inductively coupled atmospheric pressure plasma processing

Inductively coupled atmospheric pressure plasma processing (IC-APPP) is a novel polishing technique for silica-based optics to quickly mitigate the surface or subsurface damage due to its high efficiency and low cost. However, theoretically uniform removal of materials is difficult to achieve because of complex thermal effect. In this paper, numerical modelling and experimental study of global temperature field in IC-APPP is presented. First, a transient heat transfer model is established using finite element analysis to study the characteristics of temperature field distribution during the process. Then, the effects of substrate geometry and processing parameters are systematically analyzed. Finally, conformal polishing experiments are carried out to illustrate the correlation between the effectiveness of uniform removal and the temperature field. The surface shape can be maintained well with optimal processing parameters in IC-APPP.

Plasma functional polymerization of dopamine using atmospheric pressure plasma and a dopamine solution mist.

By using DBD-type atmospheric pressure plasmas and a dopamine solution mist formed by a piezoelectric module, the possibility of depositing functional polymer films showing the physical and chemical characteristics of polydopamine without breaking the functional group of the dopamine has been investigated for different plasma voltages. The higher DBD voltages up to 3.0 kV decreased the functional groups such as catechol and amine (N/C ratio) relative to dopamine in the deposited polymer by increasing the dissociation of dopamine into atoms and small molecules due to higher electron energies. In contrast, the lower DBD voltages up to 1.5 kV increased the functional group and N/C ratio of dopamine in the deposited polymer by keeping the molecular structures of the dopamine due to lower electron energies. Therefore, the polymer deposited at the lower DBD voltages showed lower contact angles and higher metal absorption properties which are some of the surface modification characteristics of polydopamine. When the metal absorption properties of the polydopamine-like film deposited using the atmospheric pressure plasma of a low DBD voltage with a dopamine solution mist were compared with other metal absorbers for Cu, As, and Cr, the polydopamine-like film exhibited superior metal absorption properties. It is believed that this atmospheric pressure plasma process can be also applied to the plasma polymerization of other monomers without breaking the functional groups of the monomers.

The Influence of Atmospheric Pressure Plasma Processing as Coating Pretreatment on Metal Material Surfaces

The Influence of Atmospheric Pressure Plasma Processing as Coating Pretreatment on Metal Material Surfaces

Fabrication of a large-aperture continuous phase plate in two modes using atmospheric pressure plasma processing

In order to fabricate a large-aperture continuous phase plate (CPP) using atmospheric pressure plasma processing (APPP) with high efficiency and precision, the position dwell mode and velocity mode were proposed and the iterative calculation method was developed for the non-constant removal rate. Two 320 mm × 320 mm × 2 mm CPPs were fabricated with two processing modes. The experiment results show that the velocity mode is capable of significantly reducing the processing time and shape error. The total processing time is decreased from 13.2 h to 9.3 h, and the surface shape error is decreased from 0.158λ to 0.119λ (λ = 632.8 nm) (root mean square).

Rapid and selective surface functionalization of the membrane for high efficiency oil-water separation via an atmospheric pressure plasma process

Oil-water separation is a worldwide challenge because of the increasing production of industrial oily wastewater and frequent oil spills. The growing environmental and economic demands emphasize the need to develop effective solutions to separate oil and water. Recently, oil-water separation methods were developed by tuning the wettability of membranes via surface functionalization. However, the industrialization of such methods remains challenging due to the easy-fouling, high cost and complex fabrication. Herein, a simple and rapid pathway to separate oil from oil-water mixtures is reported using plasma surface functionalization in an open-air environment. The fine tuning and study of the plasma process parameters enables the selective functionalization of each side of the membranes which led respectively to a superhydrophobic-superoleophilic and superhydrophobic-oleophobic sides. The successful separation, without any external force, of a 50 mL oil-water solution in 6 minutes was achieved. This work paves the way for an efficient, low cost and easily upscalable method for oil-water separation due to the high versatility of the atmospheric pressure plasma processes.

Deposition of a stable and high concentration of carboxylic acid functional groups onto a silicon surface via a tailored remote atmospheric pressure plasma process

Processes to introduce carboxylic acid functional groups onto surfaces have been widely applied in various applications, such as molecular grafting for biosensors, biocompatibility improvement, gas filter/adsorption in environmental engineering and the enhancement of interfacial adhesion between fillers (such as carbon or glass fibre) and matrices (such as epoxy resin) in composite materials. To develop an environmental-friendly process to introduce a stable and high concentration of carboxylic acid functional groups, Acrylic Acid (AAc) was deposited onto silicon wafers using remote argon atmospheric plasma processing (APP) and vapour phase grafting in a bespoke Pyrex cylindrical chamber. The chamber stands vertically with an APP nozzle installed at the top, a sample platform downstream and an AAc vapour inlet on the chamber wall. In this way, both the effect of atmospheric gas on the process and AAc monomer fragmentation during APP can be limited. Silicon specimens were exposed to a combination of plasma gas and AAc vapour. Surface wettability was evaluated by measurement of water contact angles and chemical composition was analyzed using X-ray Photoelectron Spectroscopy (XPS). The peak at the binding energy of 289.2eV, which fits with COOH components in the C1s spectrum of poly-AAc, was used to determine the contribution of the carboxylic acid groups depositing on the surface because it was absent in the C1s peak of untreated silicon wafers. A more stable and hydrophilic wettability and the binding energy peak at 289.2eV from XPS show the presence of carboxylic acid groups on the surface of samples treated with AAc vapour with the remote argon-APP. A higher intensity peak at 289.2eV was detected, compared with the surfaces activated by a conventional argon-APP in which the AAc vapour is exposed to the plasma glow, suggesting a potential, rapid and novel remote APP process for carboxylic acid functional group deposition.

(Invited) Microplasmas Technologies for Engineering of Silicon Based Quantum Dot Solar Cells

Silicon nanocrystals (Si-ncs) are fascinating candidates for numerous applications such as photovoltaic cells. Solar cell technologies are highly dependent on silicon materials and Si-ncs with optimal energy band gap present potential for further development high efficiency solar cells. Especially, the tunability of the Si energy bandgap by precisely controlling the size of Si-ncs due to quantum confinement effects is expected to increase the efficiency of third generation all silicon tandem solar cells. Among the different semiconductor nanocrystals, Si-ncs have a special value as they are generally non-toxic and have been widely used on an industrial scale. A crucial aspect for Si-ncs integration into PV devices is represented by the control of the surface characteristics. Since quantum confinement effects are observed for small nanocrystal sizes (<10 nm in diameter) and at these dimensions the Si-nc surface chemistry plays a critical role for the overall optoelectronic properties and device performance. While the use of bulky and/or long molecular passivation is frequent and it is effective in stabilizing the Si-nc properties in colloids, it is highly detrimental for transport and device-related properties. In particular, photon absorption and subsequently exciton dissociation and carrier transport suffer from the separation between Si-ncs due to the bulky surfactants and considerably decrease the performance of devices. Recently, plasma-based approaches have been successfully applied to induce non-equilibrium chemical reactions on the surface of Si-ncs directly in colloids and to achieve surface-engineered Si-ncs without any bulky surfactants. Such approaches led to the enhancement of the Si-nc transport properties and its electronic interaction with a polymer matrix [1] as well as Si-nc long-term stability in water based solutions [2]. Another important issue is the doping of the Si-ncs. A number of issues especially regarding the large impurity formation energy by doping and transport are still in controversial debate. Doping of quantum confined Si-ncs offers unique opportunities to control the bandgap and the Fermi energy level. Particularly, boron-doped and phosphorus-doped quantum confined silicon nanocrystals (Si-ncs) are surface-engineered in colloidal solution by an atmospheric pressure radio frequency microplasma a will be underlined in this talk. In particular, we have performed the surface engineering of electrochemically etched p- and n-type Si-ncs where the no-change position of the dopant in Si-ncs is assured by room temperature processing [3]. We report on how the microplasma induced surface engineering can lead to drastically different outcomes due to different doping and how this impacts the corresponding optical and transport properties [3]. Next we report on Fermi level for doped Si-ncs. Our measurements demonstrate that surface engineering can tune the band energy levels and in particular the Fermi level. Indeed, we fabricate solar cells to test p-/n-type Si-ncs and induced surface chemistry to demonstrate the resulting effects. In particular it is noted that band alignment, dissociation efficiency and transport are all directly affected by the interplay of varying surface chemistry and doping levels. We demonstrate that surface chemistries induced on the Si-ncs strongly depend on the type of dopants and implications for third generation solar cells are discussed in details [4, 5]. As a final point, surfactant free microplasma surface-engineered Si-ncs can be integrated into the device architecture to be only optically active and provide a means of effective down-conversion of blue photons into red photons leading to 24% enhancement of the photocurrent under concentrated sunlight. We also show that the down-conversion effect under 1-sun is enhanced in the case of hybrid solar cells where microplasma engineered Si-ncs are also beneficial in the active absorption layer of the solar cells.

Photocatalytic Anatase TiO2 Thin Films on Polymer Optical Fiber Using Atmospheric-Pressure Plasma.

Due to the undeniable industrial advantages of low-temperature atmospheric-pressure plasma processes, such as low cost, low temperature, easy implementation, and in-line process capabilities, they have become the most promising next-generation candidate system for replacing thermal chemical vapor deposition or wet chemical processes for the deposition of functional coatings. In the work detailed in this article, photocatalytic anatase TiO2 thin films were deposited at a low temperature on polymer optical fibers using an atmospheric-pressure plasma process. This method overcomes the challenge of forming crystalline transition metal oxide coatings on polymer substrates by using a dry and up-scalable method. The careful selection of the plasma source and the titanium precursor, i.e., titanium ethoxide with a short alkoxy group, allowed the deposition of well-adherent, dense, and crystalline TiO2 coatings at low substrate temperature. Raman and XRD investigations showed that the addition of oxygen to the precursor's carrier gas resulted in a further increase of the film's crystallinity. Furthermore, the films deposited in the presence of oxygen exhibited a better photocatalytic activity toward methylene blue degradation assumedly due to their higher amount of photoactive {101} facets.

Study on the Improvement of Hydrogen Generation Efficiency by Atmospheric Pressure Plasma Process

In this paper, the correlation between hydrogen generation from methanol molecules and the emission intensity of second positive band system from nitrogen molecules was discussed in the atmospheric pressure plasma process.BR In particular, the variation of emission intensity on SP band by changing the applied voltage, measurement position, and methanol quantities was investigated.BR First, tt was found that the hydrogen generation decreased with the recombination process between oxygen atom radicals and hydrogen atoms by adding oxygen molecules.BR Second, it was found that the hydrogen generation increased by the increase of initial methanol quantities, but the emission intensity of SP band decreased by the increase of the electron collision probability with methanol molecules.BR Finally, it was found that the emission intensity of SP band increased as to the increase of the applied voltage.

Plasma assisted deposition of free‐standing nanofilms for biomedical applications

Polymeric free‐standing nanofilms are emerging as appealing materials for many applications, including biomedical ones. This communication describes a new and versatile method to obtain self‐supported plasma‐deposited coatings. Coupling a soluble sacrificial layer with low or atmospheric pressure plasma processes, free‐standing films have been prepared. Vancomycin‐embedded bio‐composite coatings have been deposited at atmospheric pressure with an aerosol‐assisted plasma process, while PolyEthylene Oxide‐like (antifouling and cross‐linked) coatings and silver‐containing (antibacterial) nanocomposite coatings have been plasma‐deposited at low pressure. The described approach can certainly be extended to many possible free‐standing nanofilms of different nature that can be synthesized by means of customized plasma processes, and it is a valid alternative to conventional wet methods to nanofilm synthesis.