Medium Pressure Plasma Research Articles

Evaluation of plasma parameters’ impact on MRR and surface roughness in plasma polishing of fused silica: An investigation of surface characterization

Advanced fabrication methods with high efficiency are needed to meet the rising demand for precision optical components. This study proposes a plasma-based isotropic polishing method to remove fused silica's surface defects and subsurface damaged layers. Hence, a medium-pressure plasma process is developed to serve this purpose. The qualitative analysis of the plasma and atom emission spectrum explores the plasma generation and scattering of the activated fluorine (F*) radicals inside the plasma chamber. Material removal rate (MRR) and surface roughness variations with different total pressures of 5–30 mbar and substrate dimensions (i.e., varying length between 5 and 45 mm with fixed width and thickness of 5 and 2 mm) at fixed process parameters, i.e., radio-frequency power of 80 W, pressure ratio of 1:1, gas composition of 90:10, have been investigated. The highest and lowest MRR, i.e., 0.039 mm3/min and 0.0061 mm3/min, are achieved at 5 and 30 mbar total pressures on substrates having lengths 45 and 5 mm, respectively. The highest and lowest percentage change in surface roughness observed are 86.4 % and 17.1 % at 5 mbar and 30 mbar for substrates with 5 and 45 mm lengths, respectively. Surface morphology analysis shows the micro-cracks are reduced after plasma processing. The presence of elements, i.e., F and C, through EDX analysis, confirms the reactions on the plasma processed surface. Moreover, The XPS findings demonstrated that minimal radicals could be incorporated onto the fused silica surface during He/O2/SF6 plasma processing. At lower pressure, the surface roughness value increases due to the increased excitation and ionization rate caused by more interaction between the fluorine and surface atoms. The MRR increases with substrate dimension with constant pressure. Larger substrate dimensions result in increased MRR at lower pressure.

Medium Pressure Plasma Processing of Fused Silica: A Comparative Study for Material Removal Rate

Medium Pressure Plasma Processing of Fused Silica: A Comparative Study for Material Removal Rate

Design and development of medium-pressure plasma process for optical substrate finishing: A comparative study with wet chemical etching

Plasma polishing is a relatively new technique successfully applied for polishing optical materials due to its numerous advantages. It includes the high quality of the polished surface, reduced surface and sub-surface defects, high processing speed, etc. This paper investigates surface finish on fused silica using two processes, i.e., medium pressure plasma process and wet chemical etching. The experimental result shows that at a 20 mbar chamber pressure and 80 W radio frequency power, the surface finish of coarse machined and lapped fused silica substrate is better than 5 and 10 mbar. The experiments are also conducted using wet chemical etching on coarse machined and lapped polished fused silica substrates using a mixed hydrofluoric and sulfuric acid solution. However, using wet chemical etching, the percentage increase of surface roughness achieved on lapped and coarse machined surfaces is higher than in the plasma process. The percentage increase in surface roughness is 84.8 % and 98.81 % on coarse surfaces and 69.23 % and 892.3 % on lapped surfaces upto 30 μm material depth using plasma process (at 20 mbar pressure) and wet chemical etching, respectively. The surface microstructure of fused silica before and after processing is investigated using field-emission scanning electron microscopy. The result reveals that plasma polishing significantly reduced the micro-cracks and damaged layers. Energy dispersive X-ray results show the presence of elements fluorine, silicon, oxygen, and carbon on the processed surface, showing evidence of plasma reaction. Moreover, an additional element, i.e., sulfur, also appears on the processed surfaces from the solution during wet chemical etching. The sharp diffraction peaks have not been observed in X-ray diffraction patterns, indicating that the material is amorphous fused silica.

The role of plasma-induced surface chemistry on polycaprolactone nanofibers to direct chondrogenic differentiation of human mesenchymal stem cells.

Bone marrow-derived mesenchymal stromal cells (BMSCs) are extensively being utilized for cartilage regeneration owing to their excellent differentiation potential and availability. However, controlled differentiation of BMSCs towards cartilaginous phenotypes to heal full-thickness cartilage defects remains challenging. This study investigates how different surface properties induced by either coating deposition or biomolecules immobilization onto nanofibers (NFs) could affect BMSCs chondro-inductive behavior. Accordingly, electrospun poly(ε-caprolactone) (PCL) NFs were exposed to two surface modification strategies based on medium-pressure plasma technology. The first strategy is plasma polymerization, in which cyclopropylamine (CPA) or acrylic acid (AcAc) monomers were plasma polymerized to obtain amine- or carboxylic acid-rich NFs, respectively. The second strategy uses a combination of CPA plasma polymerization and a post-chemical technique to immobilize chondroitin sulfate (CS) onto the NFs. These modifications could affect surface roughness, hydrophilicity, and chemical composition while preserving the NFs' nano-morphology. The results of long-term BMSCs culture in both basic and chondrogenic media proved that the surface modifications modulated BMSCs chondrogenic differentiation. Indeed, the incorporation of polar groups by different modification strategies had a positive impact on the cell proliferation rate, production of the glycosaminoglycan matrix, and expression of extracellular matrix proteins (collagen I and collagen II). The chondro-inductive behavior of the samples was highly dependent on the nature of the introduced polar functional groups. Among all samples, carboxylic acid-rich NFs promoted chondrogenesis by higher expression of aggrecan, Sox9, and collagen II with downregulation of hypertrophic markers. Hence, this approach showed an intrinsic potential to have a non-hypertrophic chondrogenic cell phenotype.

Experimental investigations through modeling and optimization for fabrication of fused silica in medium-pressure plasma process

Due to its deterministic high material removal rate (MRR) and exceptional surface finish, the medium-pressure plasma process, which is based solely on chemical etching, has been developed as a promising fabrication method for fused silica. In the present investigation, the input parameters, such as radio-frequency power, total pressure, and processing time, are investigated and optimized for MRR and % change in surface roughness (%ΔRa) using response surface methodology. The process parameters, i.e., pressure ratio (SF6/O2) of 1:1 and gas composition (He: (SF6+O2)) of 90:10, have been kept constant during all experiments. The optimized value achieved for MRR and %ΔRa is 0.29 mm3/min and 3.4%, at RF power of 80 W, total pressure of 11.6 mbar, and processing time of 60 min. The result depicts that surface roughness marginally changes from 2.79 to 2.82 μm after plasma processing at optimum parametric conditions. Moreover, FESEM result shows that the cracks and irregularity have been reduced. EDX results reveal that elements silicon, oxygen, fluorine, and carbon appear on the plasma processed surface, showing the reaction occurring during plasma processing. Moreover, XPS results show the peaks of silicon (Si2p, Si2s), carbon (C1s), oxygen (O1s), and fluorine (F2p, F2s, F1s) on the processed surface of fused silica.

Surface characteristics of fused silica in medium-pressure plasma process

In order to obtain an ultra-smooth fused silica surface without surface damage efficiently, medium-pressure plasma processing has been design and developed. In the medium-pressure plasma polishing process, the stable plasma process of the medium-pressure plasma directly impacts surface roughness. The fluorine (F) atom, which is reactive and provided by SF6, O2, and He gas, is a very significant supplementary gas, and those gases play a significant part in the process. The medium-pressure plasma is used for polishing fused silica, and an optical emission spectrometer is used to observe the deviations in the intensity of atoms, ions, and species during processing. The experiment has been conducted at optimum conditions, i.e., gas composition (He:(SF6 + O2)) of 90:10, pressure ratio (SF6/O2) of 1:1, and RF power of 80 W. The surface roughness of the optical material, i.e., fused silica, has been marginally increased from Ra = 1.95 μm to Ra = 2.02 μm at a total pressure of 5 mbar and RF power of 80 W. Field-emission scanning electron microscopy results show that the morphology has been changed after the plasma process. The initial coarse microstructure of fused silica transforms a finer structure through plasma processing under optimal conditions. Energy-dispersive X-ray (EDX) results reveal that the elements Si and O are present at the surface before processing, and Si, O, F, and C, have been observed after processing. After plasma processing, EDX measurement confirms the presence of fluorine elements, demonstrating the interaction of the plasma with the substrate.

Investigation of MRR and surface characterization using plasma process

ABSTRACT Fused silica glass, in particular, offers a broad range of applications in optics. The present study investigates fused silica samples using non-conventional medium-pressure plasma polishing at optimum machining parameters. The polishing parameters, i.e. gas composition (He: (SF6+O2)) of 90:10, pressure ratio (SF6:O2) of 1:1, total pressure of plasma chamber of 5 mbar, and radio-frequency (RF) power of 80 W have been obtained at optimum parametric conditions. The highest material removal rate of 100.36 × 10−3 mm3/min is achieved at optimized process conditions. Even though the surface finish is slightly degraded from 0.28 µm to 0.30 µm, without any surface contamination as analyzed by the 3D optical profiler and Energy-Dispersive X-ray (EDX). Moreover, EDX analysis reveals the presence of fluorine elements after plasma processing, confirming the plasma’s interaction with the substrate. Furthermore, Raman spectroscopy results show a maximum 13% reduction in damaged and strained layers after plasma processing on the machined surface..

Landau damping of Alfvénic modes in stellarators

It is found that the presence of the so-called non-axisymmetric resonances of wave–particle interaction in stellarators (which are associated with the lack of axial symmetry of the magnetic configuration, Kolesnichenko et al 2002 Phys. Plasmas 9 517) may have a strong stabilizing influence through Landau mechanism on the toroidicity-induced Alfvén Eigenmodes (TAE) and isomon modes (Alfvénic modes with equal poloidal and toroidal mode numbers and frequencies in the continuum region) destabilized by the energetic ions. These resonances involve largest harmonics of the equilibrium magnetic field of stellarators and lead to absorption of the mode energy by thermal ions in medium pressure plasma, in which case the effect is large. On the other hand, at the high pressure attributed to, e.g., a Helias reactor, thermal ions can interact also with high frequency Alfvén gap modes (helicity-induced Alfvén Eigenmodes and mirror-induced Alfvén Eigenmodes), leading to a considerable damping of these modes. Only resonances with passing particles are considered. The developed theory is applied to various modes in the Wendelstein 7-X stellarator and a Helias reactor, and to two TAE modes in the LHD helical device.

Surface Analysis of Titanium Cleaning and Activation Processes: Non-thermal Plasma Versus Other Techniques

A medium pressure (5 kPa) dielectric barrier discharge operating in different atmospheres (air and argon) is used to develop a fast and easy way to remove adsorbed carbon contamination. Chemical and physical changes at the sample surface after plasma treatment are studied, making use of contact angle measurements, X-ray photoelectron spectroscopy analysis and atomic force microscopy measurements. The obtained results are compared with other chemical and thermal treatments typically used. This comparison shows that plasma treatment at medium pressure is able to remove up to 20 % more of the adsorbed carbon compared to the classical cleaning methods, while at the same time being less aggressive, leaving the sub-surface chemistry unchanged. Moreover, the analysis techniques give a fundamental insight in the reactions processes at the titanium surface when exposed to a medium pressure plasma.

Enhanced cell–material interactions on medium‐pressure plasma‐treated polyhydroxybutyrate/polyhydroxyvalerate

In this article, a medium-pressure DBD plasma treatment is used to improve the cell-material interaction of a polyhydroxybutyrate/polyhydroxyvalerate (PHB/PHV) film. PHB/PHV is a biodegradable natural polyester, used for different biomedical applications, including sutures, repair devices, and bone marrow scaffolds. The cell adhesion onto PHB/PHV is far less than optimal due to inadequate surface properties, and a surface modification is usually necessary to be able to use the full potential. Medium-pressure plasma treatments, in different atmospheres, are used to change the surface properties of a PHB/PHV foil. The hydrophilic character could be increased, as shown by water contact angle measurements. X-ray photoelectron spectroscopy (XPS) revealed an increased oxygen and nitrogen content. Cell culture test with human foreskin fibroblasts showed that the plasma was able to improve cell adhesion (both quantitatively and qualitatively). Both an increase in the number of adherent cells and an improved morphology were obtained after plasma treatment. After 7 days, a confluent cell layer could be observed on plasma-treated samples. The differences between the three discharge gases are negligible when looking at the improved cell-material interactions. From economical point of view, treatments in air are thus the best choice.

Plasma surface modification of polylactic acid to promote interaction with fibroblasts

In this work, medium pressure plasma treatment of polylactic acid (PLA) is investigated. PLA is a biocompatible aliphatic polymer, which can be used for bone fixation devices and tissue engineering scaffolds. Due to inadequate surface properties, cell adhesion and proliferation are far less than optimal and a surface modification is required for most biomedical applications. By using a dielectric barrier discharge (DBD) operating at medium pressure in different atmospheres, the surface properties of a PLA foil are modified. After plasma treatment, water contact angle measurements showed an increased hydrophilic character of the foil surface. X-ray photoelectron spectroscopy (XPS) revealed an increased oxygen content. Cell culture tests showed that plasma modification of PLA films increased the initial cell attachment both quantitatively and qualitatively. After 1 day, cells on plasma-treated PLA showed a superior cell morphology in comparison with unmodified PLA samples. However, after 7 days of culture, no significant differences were observed between untreated and plasma-modified PLA samples. While plasma treatment improves the initial cell attachment, it does not seem to influence cell proliferation. It has also been observed that the difference between the 3 discharge gases is negligible when looking at the improved cell-material interactions. From economical point of view, plasma treatments in air are thus the best choice.

Adhesion Strength of Electroless Copper Plated Layer on Fluoropolymer Surface Modified by Medium Pressure Plasma

We demonstrated high adhesive fluoropolymer/copper interface through combination of atmospheric pressure plasma with liquid phase self-assembly. However, there are some disadvantages in atmospheric pressure plasma technique, such as small processing area due to the localized plasma and high gas temperature. Medium pressure plasma process has some advantages over atmospheric pressure plasma systems. A large plasma volume and low gas temperature, available for surface treatment of polymer material, can be easier obtained at medium pressure than at atmospheric pressure, which can result in a higher overall productivity. In this paper, we investigated the adhesion strength of electroless copper plated layer formed on poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) surface modified by combination of medium pressure helium plasma irradiation and aminated acrylic polymer grafting. The 90° peel test result for copper plating film formed on the treated PFA films showed the adhesion strength of 0.44 N/mm without increasing the surface roughness.

The Effect of Medium Pressure Plasma Treatment on Thin Poly-ϵ-Caprolactone Layers

In this work, the effect of medium pressure plasma treatment on thin poly-ϵ-caprolactone (PCL) layers on glass plates is investigated. PCL is a biocompatible and biodegradable polymer which potentially can be used for bone repair, tissue engineering and other biomedical applications. However, cell adhesion and proliferation are inadequate due to its low surface energy and a surface modification is required in most applications. To enhance the surface properties of thin PCL layers spin coated on glass plates, a dielectric barrier discharge (DBD) at medium pressure operating in different atmospheres (dry air, argon, helium) was used. After plasma treatment, water contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to examine the PCL samples. These measurements show that the medium pressure plasma treatment is able to increase the hydrophilic character of the samples, due to an incorporation of oxygen groups at the surface and that the surface roughness is significantly decreased after plasma treatment.

Photoresist removal using an O2∕N2 medium pressure plasma jet with high speed wafer scanning: Unimplanted resist studies

The authors describe a plasma ashing system where a stationary jet of hot, activated gases removes photoresist from a scanning wafer. The jet is created by a reactant stream flowing through a 2.45GHz surface wave discharge in a 6mm quartz tube. For O2∕N2 plasmas in the medium pressure range from 20to100Torr, a luminous plasma jet emerges from the end of the discharge tube that transports both heat and reactive species to the wafer. A single scan results in a Gaussian track profile with a standard deviation of 7mm for the source-to-substrate distance of 9mm. A simple model of the ashing process, which assumes a thermally activated ash rate and Gaussian distributions for both power density and reactant flux, unifies the dependence of effective ash rate on the substrate temperature and scan speed at a constant power. The best fit activation energy at 2.5kW is 0.23eV, about half of the value found in conventional downstream ashing, implying that diffusion plays a significant role in limiting the ash rate. The peak thermal power density in a 2.5kW jet at 80Torr is 160W∕cm2, resulting in an effective instantaneous ash rate of 2.5mm∕min for a scan speed of 70cm∕s and 200°C chuck temperature. This implies that the time to clear a 1.2μm thick resist coating from a 300mm wafer is 18s.

Improving performance of multi-dimensional Particle-In-Cell codes for modelling of medium pressure plasma

In this contribution we estimate the performance of various Poisson equation solvers applied to the Particle-In-Cell plasma models. The solvers determine the practical usability of complex PIC models, especially in three dimensions. The performance is measured on 2D models with grids of various sizes, the methods studied are SOR, conjugate gradients, LU decomposition, FACR and multigrid methods. The results confirm the efficiency of the direct methods tested, namely the LU decomposition method and FACR. The advantages of using LU decomposition as a part of the multigrid method on larger grids are discussed as well.

Nano cluster assisted high rate epitaxy of silicon by mesoplasma CVD

Homoepitaxial silicon thick films have been deposited by medium pressure plasma chemical vapor deposition at rates as fast as 60 nm/s at temperatures around 700 °C. X-ray scattering measurement was attempted in situ simultaneously during deposition to capture the possible cluster formation in the plasma/substrate boundary. The scattering intensity was found to increase upon injection of SiH 4 gas to Ar plasma. Such an increase in the intensity was pronounced when the epitaxial films were achieved. These potentially suggest that nano sized Si clusters were formed in the boundary region and contributed effectively to the high rate epitaxial growth.

High rate epitaxy of silicon thick films by medium pressure plasma chemical vapor deposition

Homoepitaxial silicon thick films have been produced by medium pressure plasma chemical vapor deposition at rates as fast as 60nm∕s and at a temperature of around 700°C, with a silane gas partial pressure of 4mTorr. The continuous transition of the film structures from agglomerated to faceted columnar and to epitaxial planar structure was observed with an increase in the plasma power. The calorimetric analysis during deposition has also confirmed that the thermal boundary layer thickness between the plasma and substrate reduced with the increasing power and became comparable to the mean free path of the vapors when epitaxy was achieved at high rates. In addition, the rate for epitaxial growth was observed to increase linearly with silane gas partial pressure. These potentially indicate that less coagulated silicon atom clusters formed in the reduced boundary thickness have contributed effectively to the high rate epitaxial growth.

Simulation of the high-pressure mercury discharge lamp during the middle phase of start-up (medium mercury pressure)

This paper deals with the modelling of the medium pressure Hg and Hg-Ar positive column (Hg pressure range 5-350 Torr). The aim of this work is to simulate, in a first approximation, the middle phase of the Hg high-pressure lamp warm-up. In this approach, the main assumption is that time evolution of the discharge can be divided into a succession of stationary sub-phases characterized by the Hg partial pressure. Thus, we present here a self-consistent steady-state collisional-radiative model describing the middle sub-phase. This model includes volume recombination of Hg atomic and molecular ions, as well as several atom-atom inelastic scattering mechanisms. Calculations are carried out for both pure Hg and Hg-Ar discharges. Our results, which are in good agreement with experimental data from the literature, confirm that plasma thermalization occurs in the middle start-up phase (electron and gas temperatures become equal during this phase). Furthermore, this simulation shows the importance of different elementary processes, like atom-atom inelastic scattering, for the medium pressure plasma description. In fact, these mechanisms cannot be neglected until LTE conditions are reached.

Density and potential distributions of the diffusion plasma column in a coaxial cylindrical discharge tube

Numerical treatment of the charge density and the potential distribution has been done for a diffusion plasma column in a coaxial cylindrical discharge tube. The numerical results are as follows. (i) A homogeneous plasma exists in the centre region of the diffusion plasma. Therefore one may use this type of plasma column as a medium pressure plasma test chamber. (ii) The charge density distribution shows strong dependence on the volume recombination at the edge of the plasma column. This verifies the possibility for time-dependent measurements of the volume recombination coefficient as suggested by Chen et al. (1977). (iii) Experimental charge density and potential distributions agree well with present numerical solutions.

Plasma Diagnostics using Electrostatic Probes under Short Mean Free Path Conditions

The properties of a medium pressure plasma were inferred from electrostatic probe measurements using the theory of Zakharova et al. The results were compared with microwave measurements and the agreement was within a factor of 3.