Pseudo-Wet Plasma Mechanism Enabling High-Throughput Dry Etching of SiO2 by Cryogenic-Assisted Surface Reactions.

Manufacturing semiconductor devices requires advanced patterning technologies, including reactive ion etching (RIE) based on the synergistic interactions between ions and etch gas. However, these interactions weaken as devices continuously scale down to sub-nanoscale, primarily attributed to the diminished transport of radicals and ions into the small features. This leads to a significant decrease in etch rate (ER). Here, a novel synergistic interaction involving ions, surface-adsorbed chemistries, and materials at cryogenic temperatures is found to exhibit a significant increase in the ER of SiO2 using CF4/H2 plasmas. The ER increases twofold when plasma with H2/(CF4 + H2) = 33% is used and the substrate temperature is lowered from 20 to -60 °C. The adsorption of HF and H2O on the SiO2 surface at cryogenic temperatures is confirmed using in situ Fourier transform infrared spectroscopy. The synergistic interactions of the surface-adsorbed HF/H2O as etching catalysts and plasma species result in the ER enhancement. Therefore, a mechanism called "pseudo-wet plasma etching" is proposed to explain the cryogenic etching process. This synergy demonstrates that the enhanced etch process is determined by the surface interactions between ions, surface-adsorbed chemistry, and the material being etched, rather than interactions between ion and gas phase, as observed in the conventional RIE.

In Situ Monitoring of Etching Characteristic and Surface Reactions in Atomic Layer Etching of SiN Using Cyclic CF4/H2 and H2 Plasmas.

Cyclic atomic layer etching (ALE) of SiN with high selectivity to SiO2, utilizing a hydrofluorocarbon deposition followed by exposure to hydrogen plasma, is presented. The surface reaction mechanism and etching behavior were investigated with in situ attenuated total reflectance Fourier transformation infrared spectroscopy (ATR-FTIR) and spectroscopic ellipsometry. In the deposition step, the hydrofluorocarbon film was deposited on top of the SiN films using the CF4/H2 plasmas with varying H2 contents (33 to 85%). Subsequently, the surface-modified SiN film was exposed to a hydrogen plasma for etching. The self-limiting SiN etching was observed, where the etch depth solely depended on the F concentration of the deposited hydrofluorocarbon layer once its thickness exceeded a critical value. A high selectivity of approximately 8.6 for SiN over SiO2 was achieved. The in situ ATR-FTIR spectra revealed that during the deposition step, besides the formation of the C-H peak associated with hydrofluorocarbon deposition, the appearance of the N-H4 absorbance band indicated the formation of an ammonium fluorosilicate layer on top of SiN. In the subsequent H2 plasma etching step, both the surface modification layer and the pre-deposited hydrofluorocarbon layer were removed. The removal of the surface-modified layer and hydrofluorocarbon layer was associated with the etch rate during H2 plasma exposure. These findings indicate the importance of the formation and removal of the surface modification layer for achieving ALE of SiN. The dissociation of the hydrofluorocarbon layer by the H2 plasma released reactants that interacted with SiN, leading to the formation of a new surface modification layer. The etching process significantly slowed down once the hydrofluorocarbon deposition and surface modification layer were completely removed.

Study of optical emission spectroscopy using modified Boltzmann plot in dual-frequency synchronized pulsed capacitively coupled discharges with DC bias at low-pressure in Ar/O2/C4F8 plasma etching process.

We consider the corona model and local thermal equilibrium approximations of a real plasma to measure the electron temperature (Te) and density (ne), respectively, using the optical emission spectroscopy (OES) method in dual-frequency pulsed capacitively coupled plasmas (CCPs) in a reactive mixture of Ar/O2/C4F8 at a low operating pressure. The operation conditions such as DC continuous and synchronized were used for the study and plasma characterization for the intended plasma application such as high aspect ratio etching (HARE). We show that the present plasma conditions are dominated by a corona balance rather than the supremacy of multi-step excitation. This fact has enabled us to utilize the modified Boltzmann plot technique to evaluate the Te values. In the second method, we simultaneously used the Boltzmann and Saha equations to determine the ne value using the line intensity ratio and the value of Te. Time-resolved measurements of Te and ne were performed for completeness, and the insight of the pulsed discharge was investigated. Time evolution of ne and Te using the OES method revealed a similar trend in the change of plasma parameters, indicating electron impact ionization during the pulse on phase. It was seen that ne in the afterglow speedily decreased within a short time of ∼5 µs. Analysis suggests the formation of afterglow plasmas, which are composed of positive and negative ions with very low electron density. The results revealed that the DC-synchronized operation could be useful for plasma application such as HARE due to different plasma characteristics. It also suggests the production of ion-ion plasmas by the effective utilization of negative ions in the afterglow phase. The corona balance condition was validated in our experiments, and the results were compared with the existing literature.

Gene Expression of Osteoblast-like Cells on Carbon-Nanowall as Scaffolds during Incubation with Electrical Stimulation.

Nanostructured cell-culture scaffolds of carbon nanowalls (CNWs) were prepared by changing average wall-to-wall distances either 132 or 220 nm. Osteoblast-like cells (Saos-2) proliferated during 4 day incubation on the wider (220 nm) CNW scaffolds in the presence of electrical stimulation (ES). Differentiation gene expression levels of Runt-related transcription factor 2 (Runx2) and osteocalcin (OC) were suppressed after 10 day incubation, which indicated that the average wall-to-wall distances of the CNW scaffolds affect suppression of Runx2 and OC gene expression. This technique holds promise for controlling the differentiation of osteoblast-like cells.

Oriented Carbon Nanostructures by Plasma Processing: Recent Advances and Future Challenges.

Carbon, one of the most abundant materials, is very attractive for many applications because it exists in a variety of forms based on dimensions, such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and-three dimensional (3D). Carbon nanowall (CNW) is a vertically-oriented 2D form of a graphene-like structure with open boundaries, sharp edges, nonstacking morphology, large interlayer spacing, and a huge surface area. Plasma-enhanced chemical vapor deposition (PECVD) is widely used for the large-scale synthesis and functionalization of carbon nanowalls (CNWs) with different types of plasma activation. Plasma-enhanced techniques open up possibilities to improve the structure and morphology of CNWs by controlling the plasma discharge parameters. Plasma-assisted surface treatment on CNWs improves their stability against structural degradation and surface chemistry with enhanced electrical and chemical properties. These advantages broaden the applications of CNWs in electrochemical energy storage devices, catalysis, and electronic devices and sensing devices to extremely thin black body coatings. However, the controlled growth of CNWs for specific applications remains a challenge. In these aspects, this review discusses the growth of CNWs using different plasma activation, the influence of various plasma-discharge parameters, and plasma-assisted surface treatment techniques for tailoring the properties of CNWs. The challenges and possibilities of CNW-related research are also discussed.

Lipid droplets exhaustion with caspases activation in HeLa cells cultured in plasma-activated medium observed by multiplex coherent anti-Stokes Raman scattering microscopy.

The multiplex coherent anti-Stokes Raman scattering microscopy allowed label-free visualization of cytoplasmic lipid droplets (LDs). The LDs, which act to conserve energy storage, are usually accumulated during the normal apoptosis of HeLa cells with activation of caspase-3/7 leading to downregulation of the fatty acid catabolism pathways. During cultivating in nonthermal plasma-activated medium (PAM), while the activation of caspase-3/7 was induced, the authors found that a dynamic exhaustion of the intracellular LDs, underlying the metabolic mechanism of the PAM-induced apoptotic cell death of HeLa cells.

Intracellular-molecular changes in plasma-irradiated budding yeast cells studied using multiplex coherent anti-Stokes Raman scattering microscopy.

Interactions between non-equilibrium atmospheric-pressure plasma (NEAPP) and living cells were examined using multiplex coherent anti-Stokes Raman scattering (CARS) microscopy. Our multiplex CARS analyses revealed that NEAPP irradiation generates short-lived radicals that induce a decrease in the mitochondrial activity of budding yeast cells.

Occurrence and distribution of perfluoroalkyl acids (PFAAs) in surface water and sediment of a tropical coastal area (Bay of Bengal coast, Bangladesh).

This study reports the first evidence of perfluoroalkyl acids (PFAAs) in surface waters and sediments collected from the coastal area of Bangladesh. Fifteen target PFAAs, including C4-14-PFCAs (perfluoroalkyl carboxylates) and C4, C6, C8, and C10-PFSAs (perfluoroalkyl sulfonates), were quantified by HPLC-MS/MS. The ΣPFAAs in surface water and sediment samples were in the range of 10.6 to 46.8ng/L and 1.07 to 8.15ng/gdw, respectively. PFOA in water (3.17-27.8ng/L) and PFOS in sediment samples (0.60-1.14ng/gdw) were found to be the most abundant PFAAs, and these concentrations were comparable to or less than most other reported values, particularly those recorded from the coastal areas of China, Japan, Korea and Spain. The majority of the monitored PFAAs did not show clear seasonal variation. The southeastern part (Cox's Bazar and Chittagong) of the Bangladeshi coastal area was more contaminated with PFAAs than the southern (Meghna Estuary) and southwestern parts (Sundarbans). Industrial and municipal wastewater effluents, ship breaking and port activities were identified as potential sources of the PFAA contamination in this region. Field-based sediment water distribution coefficients (KD) were calculated and corrected for organic carbon content (KOC), which reduced the variability between samples. The values of log KD (1.63-2.88) and log KOC (4.02-5.16) were higher than previously reported values, which may indicate that the partitioning of PFAAs in a tropical coastal ecosystem is different from other ecosystems, such as temperate and sub-tropical regions. Although a preliminary environmental hazard assessment indicated that PFOA or PFOS levels do not currently exceed the acute safety thresholds, we should keep in mind that they are bioavailable and can accumulate in the food chain. Therefore, the ubiquity of PFAAs in the coastal area of Bangladesh warrants further studies characterizing their specific sources and the potential long-term risks they present to both humans and wildlife.

Trace metal contamination in commercial fish and crustaceans collected from coastal area of Bangladesh and health risk assessment.

Trace metals contamination in commercial fish and crustaceans have become a great problem in Bangladesh. This study was conducted to determine seven trace metals concentration (Cr, Ni, Cu, Zn, As, Cd, and Pb) in some commercial fishes and crustaceans collected from coastal areas of Bangladesh. Trace metals in fish samples were in the range of Cr (0.15 - 2.2), Ni (0.1 - 0.56), Cu (1.3 - 1.4), Zn (31 - 138), As (0.76 - 13), Cd (0.033 - 0.075), and Pb (0.07 - 0.63 mg/kg wet weight (ww)), respectively. Arsenic (13 mg/kg ww) and Zn (138 mg/kg ww) concentrations were remarkably high in fish of Cox's Bazar due to the interference of uncontrolled huge hatcheries and industrial activities. The elevated concentrations of Cu (400), Zn (1480), and As (53 mg/kg ww) were also observed in crabs of Cox's Bazar which was considered as an absolutely discrepant aquatic species with totally different bioaccumulation pattern. Some metals in fish and crustaceans exceeded the international quality guidelines. Estimated daily intake (EDI) and target cancer risk (TR) revealed high dietary intake of As and Pb, which was obviously a matter of severe public health issue of Bangladeshi coastal people which should not be ignored and concentrate our views to solve this problem with an integrated approaches. Thus, continuous monitoring of these toxic trace elements in seafood and immediate control measure is recommended.

Cell survival of glioblastoma grown in medium containing hydrogen peroxide and/or nitrite, or in plasma-activated medium.

Non-equilibrium atmospheric pressure plasmas generate a high electron density (on the order of 10(16) electrons per cm(-3)) using Ar gas. Culture medium in air at room temperature was plasma-irradiated for several hundred seconds. Tens of micromolar hydrogen peroxide (H2O2) and millimolar levels of nitrous ion (NO2(-)) were detected in the plasma-irradiated culture medium (plasma activated medium; PAM) and selectively induced the apoptotic death of glioblastoma tumor cells, but did not kill normal mammary epithelial cells. A similar antitumor effect was induced by spiking the medium with comparable concentrations of H2O2 and NO2(-). The PAM remained still a somewhat difference that it should also be assessed for understanding other latent mechanisms.

UV light photo-Fenton degradation of polyphenols in oolong tea manufacturing wastewater.

The UV light photo-Fenton degradation of oolong tea polyphenols in tea manufacturing effluent that color the wastewater to a dark brown has been examined. In order to elucidate the photo-Fenton degradation mechanism of oolong tea polyphenols and find the optimal dosages of the Fenton reagents, systematic study has been conducted. For the UV light photo-Fenton degradation of oolong tea effluent being 70 mg-(polyphenol) L(-1), the optimum dosages of Fenton reagents were found to be 20 mgL(-1) of total Fe and 500 mgL(-1) of H2O2. The polyphenol degradation could be divided into two stages. The polyphenols concentration rapidly decreased to around 30% of the initial concentration within 2 min and the degradation rate significantly slowed down in the subsequent stage. After 60 min of UV light irradiation, 97% polyphenol removal was obtained. The initial quick degradation of oolong tea polyphenols suggests that hydroxyl radical generated by the photo-Fenton process might preferentially attack polyphenols having high antioxidant activity by scavenging hydroxyl radicals. Almost complete decolorization of the oolong tea effluent was achieved after 80 min. About 96% mineralization of 63 mgL(-1) TOC loading was achieved within 60 min and then further mineralization was rather slow. The complete COD removal of 239 mgL(-1) COD loading was obtained after 100 min. The present results indicate that the UV light photo-Fenton degradation process can treat tea manufacturing wastewaters very effectively.

Solar photo-Fenton process for the treatment of colored soft drink wastewater: decolorization, mineralization and COD removal of oolong tea effluent.

The decolorization and mineralization of dark-brown-colored oolong tea effluent by the solar photo-Fenton process has been examined. The solar photo-Fenton process for a fine day achieved 92% decolorization after 60 min and 94% mineralization after 80 min. For a cloudy day, about 88% decolorization and 85% mineralization were obtained after 290 min. For reference the UV light photo-Fenton process was also conducted. Very similar degradation efficiencies were found between the solar and UV light photo-Fenton processes. However, the intrinsic low cost associated with abundant solar energy turned out to be more efficient in treating oolong tea effluent as compared with UV light. The decolorization and mineralization profiles under the different light intensities could be unified with the accumulated light energy instead of with irradiation time. This implies that the solar photo-Fenton process should be designed and operated on the basis of the accumulated energy rather than the reaction time. The COD removal was 99.3% after 75 min under the fine condition. This removal rate for a fine day was approximately twice as fast than that for a cloudy day and comparable to that by the UV light irradiation. The results obtained in this study suggest that the solar photo-Fenton process offers a promising technology for decolorization and degradation of oolong tea effluent.

Synergistic Formation of Radicals by Irradiation with Both Vacuum Ultraviolet and Atomic Hydrogen: A Real-Time In Situ Electron Spin Resonance Study.

We report on the surface modification of poly(tetrafluoroethylene) (PTFE) as an example of soft materials and biomaterials that occur under plasma discharge by kinetics analysis of radical formation using in situ real-time electron spin resonance (ESR) measurements. During irradiation with hydrogen plasma, simultaneous measurements of the gas-phase ESR signals of atomic hydrogen and the carbon dangling bond (C-DB) on PTFE were performed. Dynamic changes of the C-DB density were observed in real time, where the rate of density change was accelerated during initial irradiation and then became constant over time. It is noteworthy that C-DBs were formed synergistically by irradiation with both vacuum ultraviolet (VUV) and atomic hydrogen. The in situ real-time ESR technique is useful to elucidate synergistic roles during plasma surface modification.

Hydroxyl radical concentration profile in photo-Fenton oxidation process: generation and consumption of hydroxyl radicals during the discoloration of azo-dye Orange II.

Dynamic behaviors of hydroxyl (OH) radical generation and consumption in photo-Fenton oxidation process were investigated by measuring OH radical concentration during the discoloration of azo-dye Orange II. The effects of operating parameters for photo-Fenton discoloration, i.e. dosages of H(2)O(2) and Fe, initial dye concentration, solution pH and UV irradiation, on the generation and consumption of OH radicals playing the main role in advanced oxidation processes were extensively studied. The scavenger probe or trapping technique in which coumarin is scavenger of OH radical was applied to estimate OH radical concentration in the photoreactor during the photo-Fenton discoloration process. The OH radical generation was enhanced with increasing the dosages of Fenton regents, H(2)O(2) and Fe. At the initial stage of photo-Fenton discoloration of Orange II, the OH radical concentration rapidly increased (Phase-I) and the OH radical concentration decreased after reaching of OH radical concentration at maximum value (Phase-II). The decrease in OH radical concentration started when the complete discoloration of Orange II was nearly achieved and the H(2)O(2) concentration became rather low. The dynamic behavior of OH radical concentration during the discoloration of Orange II was found to be strongly linked with the change in H(2)O(2) concentration. The generation of OH radical was maximum at solution pH of 3.0 and decreased with an increase of solution pH. The OH radical generation rate in the Fenton Process was rather slower than that in the photo-Fenton process.

Noise induced oscillations in recurrent neural networks.

It has been shown that oscillations can be generated by additive Gaussian white noise in a recurrent Hodgkin-Huxley neuron model. Type 1 oscillation was induced with Stochastic Resonance (SR) by additive Gaussian noise at lower amplitudes, while Type 2 oscillation was observed at higher amplitudes. However, the mechanism of Type 2 oscillation is not clear. In this article, we test the hypothesis through computer simulations that the period of the Type 2 oscillation can be affected by temperature in a recurrent neural network in which the recurrent model is constructed by four Hodgkin-Huxley (HH) neuron models. Each HH neuron model is driven by Gaussian noise and sub-threshold excitatory synaptic currents with an alpha function from another HH neuron model, and the action potentials (spike firings) of each HH neuron model are transferred to the other HH neuron model via sub-threshold synaptic currents. From spike firing times recorded, the inter spike interval (ISI) histogram was generated, and the periodicity of spike firings was detected from the ISI histogram at each HH neuron model. The results show that the probability of spike firings in the Type1 oscillation is maximized at a specific standard deviation (S.D.) of the Gaussian white noise with SR at 6.3, 15.0 and 25.0 degrees C, while the period of the Type 2 oscillation depends on temperature. It is concluded that the Type1 oscillation can be induced by additive Gaussian white noise on the basis of a synaptic delay in the recurrent HH neuron model, whereas ISIs of the Type 2 oscillation may be determined by refractory periods of HH neuron models.