Plasma Exposure Duration Research Articles

Short and long time effects of low temperature Plasma Activated Media on 3D multicellular tumor spheroids.

This work investigates the regionalized antiproliferative effects of plasma-activated medium (PAM) on colon adenocarcinoma multicellular tumor spheroid (MCTS), a model that mimics 3D organization and regionalization of a microtumor region. PAM was generated by dielectric barrier plasma jet setup crossed by helium carrier gas. MCTS were transferred in PAM at various times after plasma exposure up to 48 hours and effect on MCTS growth and DNA damage were evaluated. We report the impact of plasma exposure duration and delay before transfer on MCTS growth and DNA damage. Local accumulation of DNA damage revealed by histone H2AX phosphorylation is observed on outermost layers and is dependent on plasma exposure. DNA damage is completely reverted by catalase addition indicating that H2O2 plays major role in observed genotoxic effect while growth inhibitory effect is maintained suggesting that it is due to others reactive species. SOD and D-mannitol scavengers also reduced DNA damage by 30% indicating that and OH* are involved in H2O2 formation. Finally, PAM is able to retain its cytotoxic and genotoxic activity upon storage at +4 °C or −80 °C. These results suggest that plasma activated media may be a promising new antitumor strategy for colorectal cancer tumors.

Growing aluminum nitride films by Plasma-Enhanced Atomic Layer Deposition at low temperatures

Aluminum nitride films have been grown by Plasma-Enhanced Atomic Layer Deposition method. It was found that at temperatures of 250 °C and 280 °C increase of the plasma exposure step duration over 6 s, as well as increase of reactor purge step duration over 1 s does not affect the growth rate, however, it affects the microstructure of the films. It was found that crystalline aluminum nitride films deposit with plasma exposure duration over 10 s and the reactor purging over 10 s. When the temperature drops the increase of reactor purge step duration and plasma exposure step duration over 20 s is required for crystalline AlN film growth.

N2/Ar plasma induced doping of ordered mesoporous TiO2 thin films for visible light active photocatalysis

This study reports the rapid and effective nitrogen doping of ordered, mesoporous TiO2 thin films using nitrogen/argon (N2/Ar) plasma. The resulting nitrogen-doped TiO2 (N–TiO2) films show significant enhancement in both visible light absorption and photocatalytic activity. The cubic ordered mesoporous TiO2 thin films are prepared via a sol–gel method using titanium tetrachloride (TiCl4) as precursor and triblock copolymer Pluronic F127 as the template. Following brief calcination, the TiO2 films are treated with N2/Ar plasma under controlled conditions of reactive gas pressure, microwave power, and plasma exposure duration. To vary the degree of nitrogen doping, the plasma exposure time was varied from zero up to 210 min. The nitrogen content of the films increases with plasma exposure duration, up to over 3 at% N. X-ray photoelectron spectroscopic (XPS) analyses and UV–vis absorbance spectra of N–TiO2 films indicate that the incorporated N atoms reduce the band gap of TiO2 and thus enhance the absorption of visible light. Finally, the visible-light photocatalytic activity of N–TiO2 films is determined from the photocatalytic degradation of methylene blue under visible-light illumination (with a 455 nm LED). The N–TiO2 films prepared by 150 min treatment show the optimum photocatalytic activity with a pseudo-first order rate coefficient of 0.24 h−1, which is six times greater than that of undoped TiO2 films. Treatment for excessive time (e.g. 210 min) leads to a decline in photocatalytic activity due to coarsening of the porous structure. The present study suggests that plasma-induced doping is a promising approach to enable the efficient incorporation of heteroatoms into surfactant-templated TiO2 thin films while maintaining their nanostructures, thereby leading to the significant enhancement of visible-light photoactivity.

Enhanced Step Coverage of TiO₂ Deposited on High Aspect Ratio Surfaces by Plasma-Enhanced Atomic Layer Deposition.

Plasma-enhanced atomic layer deposition (PEALD) provides multiple benefits compared to thermal ALD including lower possible process temperature and a wider palette of possible materials. However, coverage of high aspect ratio (AR) structures is limited due to the recombination rates of the radical plasma species. We study the limits of conformality in 1:30 AR structures for TiO2 based on tetrakis(dimethylamido)titanium (TDMA-Ti) and O2 plasma through variation in plasma exposure and substrate temperature. Extending plasma exposure duration and decreasing substrate temperature within the ALD window both serve to improve the conformality of the deposited film, with coverage >95% achievable. Additionally, the changes in morphology of the TiO2 were examined with crystallites of anatase and brookite found.

Tribological properties of chemically modified diamond like carbon films in hydrogen plasma

Diamond like carbon (DLC) films were deposited on silicon substrates by plasma enhanced chemical vapor deposition (PECVD) technique and chemically modified by H2 plasma treatment. The Raman spectra and XPS analysis, revealed a decrease in sp2C=C/sp3C–C ratio with increase in H2 plasma exposure duration up to 30min. This is due to the etching of sp2C=C sites by hydrogen ions/radicals during plasma treatment. However, this ratio increased fairly for longer H2 plasma exposure time. This might be due to the stabilization of sp2C=C bonding by passivation of film surface by hydrogenation. Friction coefficient of these films is found to decrease with an increase in sp2C=C fraction.

Erosion of tungsten-doped amorphous carbon films in oxygen plasma

Tungsten-doped amorphous carbon films with 0–9 at.% W concentration were produced by magnetron sputtering and eroded in oxygen plasmas applying different bias voltages and substrate temperatures. The partial C and W erosion rates were determined from the C and W areal density changes measured by Rutherford backscattering spectrometry (RBS). The initial C removal rate increases with increasing ion energy and temperature and decreases with increasing W concentration. For W-doped films the erosion rate decreases with increasing plasma exposure duration. At low bias voltages the erosion process stops after W accumulation at the surface, which protects the carbon underneath from further erosion. RBS and X-ray photoelectron spectroscopy suggest that the W-rich layer at the surface is carbon free and consists of porous WO 3. Biasing to 200 V leads to removal of W by physical sputtering and, therefore, inhibits the formation of the protecting W oxide layer and the C erosion proceeds.

Surface modification of POSS‐nanocomposite biomaterials using reactive oxygen plasma treatment for cardiovascular surgical implant applications

In this study, central composite design (CCD) was used to develop predictive models to optimize operating conditions of plasma surface modification. It was concluded that out of the two process variables, power and duration of plasma exposure, the latter was significantly affecting the surface energy (γ(s) ), chemistry, and topography of polyhedral oligomeric silsesquioxane-poly(carbonate-urea)urethane (POSS-PCU) films. On the basis of experimental data, CCD was used to model the γ(s) using a quadratic modeling of the process variables to achieve optimum surface energy to improve the interaction between endothelial cells (ECs). It was found that optimal water θ for EC adhesion and retention, which was reported 55° from supporting literature (equivalent to γ(s) = 51 mN/m), was easily achievable using the following experimental conditions: (1) power output at 30 W for 75 Sec, (2) 90 W for 40 Sec, and (3) 90 W for 55 Sec in oxygen. In vitro cell culture and metabolic activity studies on optimized films [as in (1)] demonstrate increased adhesion, coverage, and growth of human umbilical vein endothelial cells that were confluent over a shorter time period (<24 H) than controls. Such materials enhanced the EC response and promoted endothelialization on optimized films, thus demonstrating their use as bypass graft materials.

Effect of RF and microwave oxygen plasma on the performance of Pd gate MOS sensor for hydrogen

The combined effect of microwave and RF oxygen plasma treatment of SiO 2 surface on the hydrogen sensitivity of Pd gate MOS sensor has been studied. Nine different samples of thermally grown SiO 2 surface have been taken and treated with oxygen plasma of different microwave power (100 W, 150 W and 200 W respectively) while keeping RF power fixed (20 W) for different durations (5 min, 10 min and 15 min). Pd gate MOS sensors with these plasma treated SiO 2 surface as dielectric have been fabricated and tested for different concentrations (500–3500 ppm) of hydrogen at room temperature. It is observed that the sensitivity of the sensor increases for higher duration of plasma exposure and also with microwave power but decreases when the sensor is treated with 200 W microwave power for 10 min and 15 min durations. The sensor treated with oxygen plasma of 200 W microwave power for 5 min duration exhibited the highest hydrogen sensitivity (74.4%). Fixed oxide charge density has also been evaluated as a function of exposure time for varying microwave power. Surface morphology of plasma treated SiO 2 surfaces was studied by AFM to have the estimation of porosity. The high sensitivity can be attributed to the fact that oxygen plasma treatment provides the availability of higher number of adsorption sites and modification in the surface state density i.e. surface state density increases for plasma treated sensors.

Effect of Processing Parameter Changes on the Adhesion of Plasma-treated Carbon Fiber Reinforced Epoxy Composites

Atmospheric plasma treatment for the surface preparation of adhesively bonded composite joints appears promising as a replacement to current surface preparation techniques. However, questions remain regarding the sensitivity and optimization of various plasma processing parameters on final composite bond properties. In this study, we continue to investigate how plasma surface treatment processing variables ultimately affect the surface chemistry and bonding behavior of a graphite-epoxy composite. The plasma power level, the working distance of the plasma head, the carrier gas (helium) flow rate, the duration of plasma exposure, and the active gas (oxygen) concentration within the plasma were varied and correlated to surface chemistry variations using X-ray photoelectron spectroscopy (XPS). The carboxyl concentration on the surface was then measured as a function of these changes and correlated to lap shear strengths. In addition, samples were monitored using XPS to evaluate the decay behavior of the surface treatment as a function of time. Treated specimens in both inert and air environments exhibited similar decay profiles. Large changes were not observed until after 24 days of out-time. The effects of plasma treatment, duration of plasma exposure, and out-time on the crack delamination resistance (GIC) of bonded parts were assessed. G IC measurement indicated that solvent wiped bonded specimens exhibited a purely adhesive failure with unstable crack growth. Specimens with abrasion treatment exhibited reduced performance with cracks initiated in the adhesive traveling through both the adhesive-composite interface as well as the outer surface plies of the composite substrate. We believe damage to the composite substrate due to surface preparation caused this failure mode. On the other hand, plasma-treated specimens exhibited consistent failure modes for all treatments above 12 passes. The failures were entirely cohesive with the very high bond strength promoting crack propagation only within the adhesive. The GIC values indicated that the plasma-treated composites were two times as resistant to fracture as conventionally prepared specimens.

Relaxation of process induced surface stress in amorphous silicon carbide thin films using low energy ion bombardment

The surface stress in amorphous silicon carbide (a-SiC) thin films has been modified after processing. We show that low energy argon ion bombardment in an inductively coupled plasma reactor causes relaxation of the induced surface stress. Cantilever beams of variable length (25–150μm) have been fabricated and their deflection profiles have been compared before and after ion bombardment using white light interferometry. Our experiments show that the ion bombardment relaxes the stress gradient of the cantilever beams. Both the duration of plasma exposure as well as the bias voltage contribute to relaxing the stress gradient in the a-SiC thin film.

Surface Modification by Low-Pressure Plasma of Polyamide 12 (PA12). Improvement of the Water Barrier Properties

Polyamide 12 (PA12) films have been modified by CF4 and CF4+H2 (50/50 v/v) microwave plasma with different treatment times. The surface modifications have been followed versus plasma exposure duration by water contact angle measurements and atomic force microscopy (AFM) Pervaporation measurements were carried out to characterize the effects of these plasma treatments on water transport through PA12 films. From these measurements, water permeability was determined for each duration time of treatment. The efficiency of these plasma treatments in reduced permeability is compared. For both plasma treatments (CF4 and CF4+H2), the analysis of the experimental data shows an increase and then a decrease of the permeability coefficient P with treatment durations. These observations are related with the evolution of the surface versus treatment time. From all these experimental results, it is clearly shown that the barrier effect to water in plasma-treated layers of PA12 is improved significantly, especially with CF4.

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd1−yZnyTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H2 plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 105 cm−2 have been achieved. ECR Ar/H2 plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.

Modification of Active Carbon by Hydrophobic Plasma Polymers: II Fabric Substrates

Active carbon, used in various types of chemical protection devices, can absorb an extremely wide range of hazardous compounds. Unfortunately, this “universal” absorption includes ubiquitous water molecules, which can result in saturation and premature loss of efficacy. The purpose of this work has been to treat active carbon-impregnated fabric with appropriate low-pressure plasmas, so as to render the carbon surface “hydrophobic”, while minimally affecting its ability to absorb toxic gases or vapors (simultated here by CCl4). The best results have been achieved using plasma polymerization of organosilicon (PP-HMDSO) thin films onto the fabric surface: This has been done in a pilot-scale microwave plasma reactor system, designed for treating continuously-moving flexible web materials up to 30 cm in width. Under “optimal” treatment conditions, a plasma exposure duration of 10 s (PP-HMDSO film thickness, d ∼ 40 nm) is found to be sufficient to reduce water absorption by 85%, while the corresponding reduction in CCl4absorption is small (<20%). The treatment demonstrates long-term stability, and it holds promise for commercial implementation on existing roll coaters.

Controlled nitrogen incorporation at the gate oxide surface

Nitrogen has been incorporated selectively at the top surface of a conventional thermal gate oxide by nitridation with a remote He–N2 plasma at low temperatures, 23 and 300 °C. On-line Auger electron spectroscopy (AES) has been used to characterize the process. A peak shift in the Si-LVV feature establishes that the nitrogen is bonded to the silicon. The concentration of nitrogen can be varied by a combination of substrate temperature and duration of plasma exposure. Ex situ glancing-angle x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) confirm that the nitrogen is confined to the immediate vicinity of the surface. Rapid thermal annealing (RTA) of the nitrided oxide at 900 °C in N2 and N2O does not change the N content.

Fabrication of a continuous wettability gradient by radio frequency plasma discharge

Radio frequency (RF) plasma discharge in atmospheres of ammonia, oxygen, and sulfur dioxide was used to create a continuous change in wettability on the surfaces of polyethylene, polystyrene, polydimethylsiloxane, and polytetrafluoroethylene. These continuous changes, referred to as wettability gradients, were produced by exposing the polymers to the RF plasma in a box with a cover which opened at a constant velocity. Contact angles of water on the derivatized polymer surfaces decreased monotonically as the time of plasma exposure increased. A wide range in wettability was produced by manipulating the RF power, the gas species, and the duration of plasma exposure. The production of such surfaces has promising application in surface science because a continuous spectrum of surface energies or surface chemistries can be produced on one sample at one time.