Chlorine Plasma Research Articles

Cyclic Atomic Layer Etching of PdSe2

AbstractThe unique characteristic of 2D transition‐metal dichalcogenide (TMD) semiconductor materials such as PdSe2 is the ability of the bandgap to vary on the basis of the number of layers present. This variation results in a broad bandgap range spanning from semi‐metallic to semiconducting materials, underscoring the significance of precisely controlling the material thickness. In this study, the thicknesses of chemical vapor deposited PdSe2 films are precisely controlled layer‐by‐layer via cyclic atomic layer etching (ALE), which comprised a radical adsorption step generated by oxygen and chlorine plasmas, followed by a desorption step with an organic solvent vapor such as formic acid. Cyclic etching with one monolayer of PdSe2/cycle is achieved using both types of adsorption gases while maintaining the ratio of Se/Pd underlying PdSe2 at ≈2. This cyclic ALE method is also applicable for smoothing TMD material surfaces by isotropically removing individual rough surface layers. Because this etching method only utilizes chemical reactions with radicals generated by plasmas and organic vapors, it is not only unaffected by the physical damage caused by the irradiation of energetic particles such as ions in plasmas, but also is applicable to 3D structure processing.

Fabrication of high aspect ratio AlN nanopillars by top-down approach combining plasma etching and wet etching

This study proposes a top-down approach for fabricating high aspect ratio AlN pillars with m-oriented nonpolar sidewalls, which will serve as the first building block for the fabrication of core-shell UV-LEDs. Such structures are achieved through a two-step process, combining a chlorine plasma etching, followed by a wet chemical etching with KOH. In this work, the mechanisms driving the AlN etching in chlorine plasma are discussed. In particular, we highlight the impact of the ratio between ion flux and radical flux on AlN etch rates, pillar profiles, and crystal orientation-dependent etching. We also identify two mechanisms of passivation layer formation on the pillar sidewalls that contribute to the pattern slope: redeposition of the carrier wafer etch by-products and Aluminium line of sight redeposition, both phenomena are also driven by the ionic bombardment. Low ionic bombardment (either low ion over radical flux ratio or low ion energy) has been identified as plasma conditions allowing the patterning of anisotropic AlN pillars but with the formation of a-nonpolar facets. Due to the inability to obtain m-oriented sidewalls using Cl2 plasma etching alone, we show that the use of a KOH wet etching treatment allows verticalizing the pillars and revealing smooth m-facets on the pillars’ sidewalls if suitable m-oriented hard mask is used. In this wet etching step, we highlight the key role played by the hard mask and its initial shape during the wet etching if straight and m-oriented pillars are desired.

Defect Engineering for Enhanced Electrocatalytic Oxygen Reaction on Transition Metal Oxides: The Role of Metal Defects.

Metal defect engineering is a highly effective strategy for addressing the prevalent high overpotential issues associated with transition metal oxides functioning as dual-function commercial oxygen reduction reaction/oxygen evolution reaction catalysts for increasing their activity and stability. However, the high formation energy of metal defects poses a challenge to the development of strategies to precisely control the selectivity during metal defect formation. Here, density functional theory calculations are used to demonstrate that altering the pathway of metal defect formation releases metal atoms as metal chlorides, which effectively reduces the formation energy of defects. The metal defects on the monometallic metal oxide surface (Mn, Fe, Co, and Ni) are selectively produced using chlorine plasma. The characterization and density functional theory calculations reveal that catalytic activity is enhanced owing to electronic delocalization induced by metal defects, which reduces the theoretical overpotential. Notably, ab initio molecular dynamics calculations, ex situ XPS, and in situ ATR-SEIRAS suggest that metal defects effectively improve the adsorption of reactive species on active sites and enhance the efficiency of product desorption, thereby boosting catalytic performance.

Gas heating by inductively coupled low-pressure chlorine process plasmas

The mechanism of gas heating in low-pressure inductively coupled chlorine plasma is analyzed using a self-consistent two-dimensional axisymmetric fluid plasma model that is coupled with the compressible Navier–Stokes equations. For gas pressures of 10 and 20 mTorr and the discharge power in the range 0.1–1.3 kW, the main reactions contributing to gas heating were the ion–ion recombination reactions and the quenching of electronically excited chlorine atoms. At the same time, the energy released by the electron impact dissociation reaction of molecular chlorine is negligible due to its high degree of dissociation within the plasma bulk. The comparison between the results of our simulations and the fitting equation proposed in the literature show qualitative agreement, although there is significant quantitative discrepancy.

Silicon etching by chlorine plasma: Validation of surface reactions mechanism

The objective of this paper is the validation of a surface reaction mechanism for silicon etching in low-pressure chlorine plasmas. We employ a quasi-one-dimensional fluid model to model the experimental conditions of Khater and Overzet [Plasma Sources Sci. Technol. 13, 466 (2004)]. This model couples self-consistently the plasma fluid equations with the surface reaction mechanism derived from the available literature. Based on the comparison between the experiments and modeling results, the best set of etch yield parameters is proposed for the conditions typical for industrial plasma processing. The influence of these etch yield parameters on the gas-phase plasma is also discussed.

Limitations of the independent control of ion flux and energy distribution function in high-density inductively coupled chlorine plasmas

Using a self-consistent plasma model coupled with Maxwell's equations, the limitations of independent control of ion fluxes and their energy distribution functions extracted from the high-density inductively coupled chlorine plasma are studied. Two extreme cases of discharge power are considered: 100 W and 1 kW. We find that in the low-power case, plasma is mainly generated by electromagnetic waves while the radio-frequency biased electrode primarily enables plasma ion extraction. Therefore, the ion fluxes and distribution functions are controlled independently. For the high-power case of 1 kW, the bias electrode significantly contributes to plasma generation but has only a small effect on sheath voltage. As a consequence, independent control of ion fluxes and distribution functions becomes impossible. Namely, the increase in the power driving the radio-frequency electrode leads to the increase in the ion fluxes but has little effect on their energy and angular distributions.

Chlorine-Assisted Growth of Epitaxial InGaN and AlGaN Crystals at Low Temperatures Using Plasma-Enhanced Chemical Vapor Deposition

The chlorine-assisted growth of epitaxial indium gallium nitride (InGaN) and aluminum gallium nitride (AlGaN) crystals was achieved with high reproducibility at the low temperature of 600 °C in the plasma-enhanced chemical vapor deposition (PECVD) reactor via the reaction between the metal trichlorides with high vapor pressures and nitrogen plasma. After adding hydrogen gas to the growth processes of InGaN and AlGaN, the problems for the etching of chlorine plasma on the as-grown InGaN crystals and the appearance of crystal planes different from (002), derived from the worse and the better bonding abilities of materials, respectively, were solved effectively. Inside AlGaN crystals, the phase separation of gallium nitride (GaN) and aluminum nitride (AlN) can be avoided by controlling the amount of precursor aluminum. The epitaxial InGaN and AlGaN crystals have the same (002) crystal plane as the GaN(002) bottom layers and exhibit the same ⟨0001⟩ crystallographic growth direction along the axial direction of GaN microrods, representing the consistent growth orientation of the epitaxial crystals. The process temperatures of InGaN and AlGaN can be greatly decreased using chlorine gas and plasma so that the related applications of low-temperature growth may be expanded drastically in the future.

The Importance of Taking into Account the Heterogeneous Recombination of Atoms when Studying the Kinetics of Copper Etching in Chlorine Plasma

The Importance of Taking into Account the Heterogeneous Recombination of Atoms when Studying the Kinetics of Copper Etching in Chlorine Plasma

Influence of the Cl2 etching on the Al2O3/GaN metal–oxide–semiconductor interface

Controlling the plasma etching step involved in metal-oxide-semiconductor high-electron-mobility-transistor (MOSHEMT) GaN fabrication is essential for device performance and reliability. In particular, understanding the impact of GaN etching conditions on dielectric/GaN interface chemical properties is critically important. In this work, we investigate the impact of the carrier wafers (Si, photoresist, SiO2, and Si3N4) used during the etching of GaN in chlorine plasma on the electrical behavior of Al2O3/n-GaN metal–oxide–semiconductor (MOS) capacitors. X-ray Photoelectron spectroscopy (XPS) analyses show that the Al2O3/GaN interface layer contains contaminants from the etching process after the Al2O3 deposition. Their chemical nature depends on the plasma chemistry used as well as the chemical nature of the carrier wafer. Typically, Cl and C are trapped at the interface for all substrates. In the particular case of Si carrier wafer, a significant amount of SiOx is present at the Al2O3/GaN interface. The capacitance–voltage (C–V) characteristics of the MOS capacitors indicate that the presence of Si residues at the interface shifts the flat band voltage to negative values, while the presence of Cl or C at the interface increases the hysteresis. We demonstrate that introducing an in situ plasma cleaning treatment based on N2/H2 gas, before the atomic layer deposition, allows the removal of most of the residues except silicon and suppresses the hysteresis.

On the chemistry mechanism for low-pressure chlorine process plasmas

A chemical reaction mechanism of chlorine plasma under low-pressure conditions that is widely used in the literature is validated against the experimental data of Y. Wang and J. K. Olthoff [J. Appl. Phys. 85, 6358 (1999)] for an inductively coupled plasma reactor. The model used in the present study is a self-consistent two-dimensional fluid plasma model coupled with Maxwell’s equations. The quantities of interest in the plasma are the fluxes and energy distribution functions of Cl2+ and Cl+ ions. We find that the charge exchange reaction between Cl+ and Cl2 that is typically included in chlorine plasma reaction mechanisms results in poor predictability of the model compared to experiments. Neglecting this reaction allows for a correct prediction of the dominant ion species in the low-pressure chlorine plasma and dependence of their fluxes on the gas pressure. Additionally, neglecting the charge exchange reaction allows for a rather accurate prediction of ion energy distribution functions at the grounded electrode. Overall, we conclude that the rate coefficient of the charge exchange reaction between Cl+ and Cl2 reported in the literature significantly exceeds what may in fact explain the role of this process in a low-pressure plasma discharge.

Influence of electron energy distribution on fluid models of a low-pressure inductively coupled plasma discharge

The aim of the present paper is to examine the influence of assumption on the electron energy distribution function on the relation between the plasma potential and the electron temperature for both electropositive (argon) and electronegative (chlorine) plasmas. A one-dimensional fluid model is used for simplicity although similar results were obtained using a self-consistent two-dimensional fluid model coupled with the Maxwell's equations for inductively coupled plasmas. We find that for electropositive plasma only a bi-Maxwellian electron energy distribution function provides reasonable results compared to measurements in low-pressure inductively coupled plasmas, namely, the increasing plasma potential for increasing electron temperature. For electronegative plasma, the plasma potential is an increasing function of the electron temperature for all electron distributions considered in the model. However, the scaling factors do not agree with the conventional plasma theory. We explain these results by the deviation of electrons from a Boltzmann distribution, which is due to non-equilibrium and non-local nature of plasma at the low-pressure conditions.

Reference intervals and physiological variations of the macro-mineral plasma concentrations in Ouled Djellal ewes

Abstract. This study aimed to establish values and reference intervals of macro-mineral plasma concentrations, as well as the variations of these variables depending on the physiological stages of Ouled Djellal ewes, reared in the Northeast of Algeria. Six plasma macro-mineral elements: calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na), potassium (K), and chlorine (Cl), were analyzed in blood samples belonging to one hundred thirty-four (n=134) healthy Ouled Djellal ewes (2-5 years old). Obtained levels were statistically analyzed using Reference Value Adviser software in order to establish the reference intervals. In addition, a follow-up study using thirty (30) ewes was carried out to determine the variations of these same parameters depending on the animals’ physiological stages. Animals were subjected to blood sampling during seven periods of their reproductive cycle (dry period; early gestation; mid-gestation; late gestation; early lactation; mid-lactation; after weaning). Reference intervals values for the studied macro elements were: Ca (66-106.7 mgl/L), P (30.5-85 mg/L), Mg (17-32 mg/L), Na (132.6-165 mEq/L), K (3.8-6.2 mEq/L) and Cl (98-123.9 mEq/L). The follow-up study showed significant variations of these macro-elements according to the animal’s reproductive stages. In late gestation, both calcium and magnesium levels increased significantly; however, they showed a significant decrease in early lactation. Phosphatemia was significantly higher in both dry and empty ewes. The lowest values of sodium and potassium were recorded at the beginning of lactation. Chlorine plasma levels decreased very significantly in mid-lactation. Such results provide baseline information that permits monitoring the impact of various mineral disturbances in this breed and checking the animal’s health status depending on its physiological stage.

Low-temperature plasma atomic layer etching of molybdenum via sequential oxidation and chlorination

In this study, an atomic layer etching (ALE) process for molybdenum was developed in two steps: plasma oxidation and plasma chlorination. In the plasma oxidation step, molybdenum was oxidized with oxygen plasma to form molybdenum oxide. As the plasma oxidation time increased, the atomic ratio of O-to-Mo, determined by x-ray photoelectron spectroscopy, increased, and then saturated to a value of 2.3. The oxidation depth of molybdenum was found to increase with increasing oxidation temperature—from 3.0 nm at 40 °C to 22.0 nm at 300 °C. It also increased with increasing RF (radio frequency) power—from 2.0 nm at 5 W to 5.5 nm at 25 W. In the plasma chlorination step, it is believed that molybdenum oxide was removed from the surface by forming molybdenum oxychloride (MoOCl2, MoOCl4, and MoO2Cl2) in chlorine plasma in the temperature range of 40–300 °C. The etch per cycle (EPC) continuously increased at temperatures above 100 °C; however, at temperatures below 40 °C, it was saturated. The RF power increased the EPC from 2.2 to 5.8 nm/cycle in the range of 5–25 W. It was found that the removal depth matched the oxidation depth at each RF power in ALE at 40 °C. The atomic composition of molybdenum after ALE was almost identical to that before ALE. This study demonstrates that the ALE of molybdenum at 40 °C can be realized by sequential plasma oxidation and chlorination.

Application of plasma chemical treatment for manufacturing of instrument microstructures based on gallium nitride

The epitaxial layers of n-n+-GaN were processed by plasma-chemical etching using a Sentech SI 500 unit equipped with an inductively coupled plasma source. The regimes of gallium nitride processing in chlorine plasma have been established, which make it possible to remove epitaxial layers of the semiconductor down to a depth of 10 μm with a smooth surface. Based on the obtained processing results, prototype samples of Schottky diode microstructures with quasi-vertical contact geometry were manufactured. The effect of pretreatment on the characteristics of instrument microstructures is demonstrated.

Etching of iron and iron–chromium alloys using ICP-RIE chlorine plasma

The dry etching process of Fe, Cr and Fe–Cr alloys under a chlorine-based plasma is studied. The objective is to create new surface functionalities. The approach combines an experimental study of an ICP (Inductively Coupled Plasma) reactor with the development of a multi-scale etching model including kinetic, sheath and surface models. The results from plasma etching of substrates made of Fe, Cr and Fe–Cr alloys are presented. Optical emission spectroscopy and interferometry measurements show strong modifications of the plasma when Fe or Cr samples are present in the reactor. It is shown that Fe is easier to etch than Cr. The study highlights the role of chemical etching by the formation of volatile products such as FeCl3. The Cr content in Fe–Cr alloys has a strong impact on both the lateral and vertical etch rates, as well as on the roughness along the profile. For Fe-Cr alloys, the experimental and calculated values of etch rate are very similar. The concept of hard zones is introduced to get a better agreement between simulation results and experimental ones. This good agreement demonstrates the capability of the developed simulator to implement new phenomena.

Sheet Resistance Reduction of MoS₂ Film Using Sputtering and Chlorine Plasma Treatment Followed by Sulfur Vapor Annealing

Sheet resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> ) reduction of a-few-layered molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) film using sputtering is investigated in this study. To enhance the carrier density, chlorine (Cl2) gas excited by inductively coupled plasma is introduced as a substitute for sulfur. To electrically activate the Cl dopants and simultaneously prevent out-diffusion of sulfur, a furnace annealing was performed in sulfur-vapor ambient. Consequently, the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> in the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film with the Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma treatment remarkably reduced by one order lower than that without one, because of the activation of Cl dopants in the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film.

X-ray-line coincidence photopumping in a potassium-chlorine mixed plasma

Exploiting the multiple long pulse capability and suite of x-ray diagnostics of the Orion laser, we have set out to explore line coincidence photopuming---the enhancement in population of an atomic level brought on by resonant absorption of x rays from a different emitting ion. Unlike previous work, the two ions are in the same plasma and so the experiment is an x-ray analog of the well-known Bowen resonance fluorescence mechanism that operates in astrophysical situations in the optical region. Our measurements have shown enhanced fluorescence in a chlorine plasma, attributable to line coincident photopumping from co-mixed potassium ions. To detect this relatively low signal-to-noise phenomenon, the data from multiple shots are combined, and the statistical method of bootstrapping is used to assign a confidence value to the measured enhancement, resulting in an estimate of the enhancement of $39{\ifmmode\pm\else\textpm\fi{}}_{18}^{16}%$ compared to the null case, where no pumping occurs. The experimental results have been compared to coupled radiation-transport and radiation hydrodynamics simulations using the cretin code together with the nym radiation hydrodynamics model and agreement has been found, with the simulations also predicting modest enhancement.

Evidence for anti-synergism between ion-assisted etching and in-plasma photoassisted etching of silicon in a high-density chlorine plasma

Etching of p-Si in 60 mTorr 10%Cl2/90%Ar Faraday-shielded inductively coupled high density plasmas was investigated under both ion-assisted etching (IAE) and photoassisted etching (PAE) conditions. Real-time etching rates and after-etching Si surface chemical compositions were obtained by laser interferometry and vacuum-transfer x-ray photoelectron spectroscopy (XPS), respectively. Precisely controlled ion energy distributions (IEDs) were generated by applying pulsed negative DC bias on the conductive sample stage. Above a 36 eV threshold at a total flow rate of 250 SCCM, the IAE rate increased with the square root of the ion energy. In contrast to the DC bias, etching under RF bias did not exhibit a threshold ion energy because of the wide IED. XPS spectra revealed that the surface layer under PAE conditions had a significantly lower chlorine content, composed of only SiCl. Under IAE conditions, however, silicon dangling bonds (Si•), SiCl2, and SiCl3 were found on the surface, in addition to SiCl, with a relative abundance of SiCl &amp;gt; SiCl2 &amp;gt; SiCl3. The absence of higher chlorides and Si• under PAE conditions suggested that vacuum ultraviolet photons and above threshold-energy ions interact with the surface very differently. By varying the duty cycle of the pulsed DC bias, it was found that the IAE rate scaled with the energetic ion dose, but only for low duty cycles. For higher duty cycles, the apparent IAE yield fell off with an increasing Cl coverage on the surface, as the duty cycle went up, which pointed to a negative synergy (antisynergism) between PAE and IAE as the explanation. This antisynergism was further supported by the observed decrease of the total etching rate with an increasing period of the pulsed DC bias. A plausible mechanism is that increasing the pulsing period causes more near-surface damage, creating more recombination centers that lead to a higher loss rate of electron-hole pairs through recombination, thereby reducing the PAE rate.

Metalorganic vapour-phase epitaxy of AlGaN/GaN heterostructures on chlorine plasma etched GaN templates without buried conductive layer

In this work we present approach that allows regrowth of AlGaN/GaN heterostructures on plasma etched GaN templates without occurrence of buried conductive layer. Discussion about the influence of RIE (reactive ion etching) process on the properties of GaN surface is followed by presentation of experimental work results focused on reconstruction of GaN template surface after etching in chlorine plasma in order to growth AlGaN/GaN heterostructure without parasitic channel. Analysis of GaN surface treatment after RIE process using 10% aqueus HF solution and low temperature GaN (LT-GaN) nucleation layer is carried out, including SEM (scanning electron microscopy) imaging and SIMS (secondary ion mass spectroscopy) profiling. The AlGaN/GaN heterostructures with the thickness as low as 250 nm deposited on plasma etched GaN templates with sheet resistance under 600 Ω/□ and good uniformity were fabricated using this approach. Presented method can be used in fabrication of current aperture vertical electron transistor (CAVET) structures with low leakage currents.

Effects of Chlorine Water and Plasma Gas Treatments on the Quality and Microbial Control of Latuca indica L. Baby Leaf Vegetable during MA Storage

Effects of Chlorine Water and Plasma Gas Treatments on the Quality and Microbial Control of Latuca indica L. Baby Leaf Vegetable during MA Storage