Diagnostics of reactive RF plasmas

Experimental characterization of the plasma state as well as the neutral gas phase in a low pressure discharge is presented. A radiofrequency supply produces plasma driving an electrical discharge in a gas mixture through a resonant inductive coupling. The plasma state has been characterized by means of electrostatic probes, optical emission spectroscopy. Mass spectroscopy of the neutral and of the charged species has been performed too, sampling the gas phase inside the discharge. Results on Ar/SF₆ plasmas are presented to study the effect of argon doping on the production of reactive atomic fluorine in the discharge. More efficient application of Ar/SF₆ plasmas to surface fluorination of polymers could be expected in view of these results.     

Black Silicon formation using dry etching for solar cells applications

A study on the formation of Black Silicon on crystalline silicon surface using SF₆/O₂ and SF₆/O₂/CH₄ based plasmas in a reactive ion etching (RIE) system is presented. The effect of the RF power, chamber pressure, process time, gas flow rates, and gas mixtures on the texture of silicon surface has been analyzed. Completely Black Silicon surfaces containing pyramid like structures have been obtained, using an optimized mask-free plasma process. Moreover, the Black Silicon surfaces have demonstrated average values of 1% and 4% for specular and diffuse reflectance respectively, feature that is suitable for the fabrication of low cost solar cells.     

Multi-walled carbon nanotubes grow under low pressure hydrogen,air, and argon ambient by arc discharge plasma

Multi-walled carbon nanotubes (MWCNTs) were grown on cathode deposit by arc discharge plasma under H₂, Ar, and air ambient environment. The influence of ambient gas pressure on the structure and physical properties of carbon nanotube were compared. Herein, we highlight the influence of ambient environment and pressure to grow high quality carbon nanotubes. Field emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction were used for structural characterization and yield determination. The result revealed that background gas and pressure were crucial factor for growing highly crystalline and highly graphitic with I_D/I_G ratio 0.237 obtained for MWCNTs' synthesized in H₂ environment with extreme low defects.     

Procedure for Measuring the Electrophysical Characteristics and Kinetics of Electronegative Gas

The experimental procedure and basic processes affecting the dynamics of radiation-induced electrical conductivity in electronegative (SF₆) gas are analyzed. It is demonstrated that in the range of SF₆ gas pressure from 10 to 100 torr the measurement error of this procedure does not exceed 10%.     

GaN reactive ion etching using SiCl<Subscript>4</Subscript>:Ar:SF<Subscript>6</Subscript> chemistry

Reactive ion etching with SiCl₄:Ar:SF₆ mixtures of gallium nitride epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) has been studied. The effects of several factors such as gas mixture, chamber pressure, and drive power on the etch rate and etched profile have been investigated. A strong dependence of both properties with the amount of SF₆ present in the mixture has been found. High etch rates (> 50 nm/min) and controllable sidewall angles and smoothness have been achieved.     

Absorption and emission spectra of the YBCO laser plume

The plasma produced during laser ablation deposition of thin film YBCO has been studied by optical emission spectroscopy. There is evidence of increased YO band emission in the range 590–625 nm as the ambient oxygen gas pressure confining the plume is increased in the range 30–200 m Torr. Temporal profiles show that close to the target the plume is insensitive to ambient oxygen pressure. It is deduced that the optical emission here is excited by electron impact excitation. Further away from the target there is evidence that two distinct processes are at work. One is again electron excitation; the emission from this process decreases with distance because the expanding plume cools and collisions become less frequent in the expanding gas. The second is driven by oxidation of atomic species expelled at high speeds from the target. The main region of this activity is in the plume sheath where a shock front ensures heating of ambient O₂ and reaction of monatomic plasma species to form oxide in an exothermic reaction. Spatial mapping of the emission demonstrates clearly how increasing oxygen gas pressure confines the plasma and enhances the emission intensity from the molecular YO species ejected from the target in a smaller region close to the target. Ba⁺ is observed as a dominant species only very close to (within 1 mm of) the target. Absorption spectra have been taken in an attempt to examine ground state and cool species in the plume. They reveal the quite surprising result that YO persists in the chamber for periods up to 1 msec. This suggests an explanation for the recent report of off-axis laser deposition in terms of simple condensation. Previously, quasi-ballistic transfer of material from target to substrate has been considered the only significant process.     

Optical measurement in carbon nanotubes formation by pulsed laser ablation

Carbon nanotubes (CNTs) were produced by laser ablation of a graphite composite target in argon and nitrogen ambient gas. To investigate the effect of nitrogen gas on CNTs formation, the plasma plume was examined using optical emission spectroscopy. The vibrational temperature of C₂ molecules was estimated by fitting of a Swan band spectrum. The temperature in N₂ ambient gas is lower than that in Ar ambient gas. In a nitrogen atmosphere, the spectrum intensity of C₂ Swan band was enhanced and CN violet system was also observed. Soot collected in the reaction tube was observed using FE-SEM and TEM. The soot deposited in the nitrogen gas contained more bundled CNTs than those in Ar ambience.     

Effect of a new low pressure SF6 plasma sterilization system on polymeric devices

A low pressure RF remote plasma, with low temperature, was used as new sterilization system. This system was designed especially for thermo-sensitive materials. SF₆ plasma has been found to totally inactivate all investigated microorganisms within three minutes. The effect of the SF₆ plasma treatment (soft conditions) and following the aging on three polymers films has been investigated.The evolution of surface after plasma treatment was studied by several techniques. FTIR was used to follow the degradations and changes in the bulk polymers. The SEM images showed the etching and grafting on the polymers surfaces. The decreasing of the wettability in the treated surfaces indicates an increasing in surface hydrophobicity. The decreasing of C and O atoms on the surface and the developing of non-polar sites (F functionalities) were observed by XPS analysis. The surface modifications were found to be permanent.     

Large area ashing process using an atmospheric pressure plasma

We present an atmospheric pressure plasma processing for ashing photo-resist (PR) layer in the flat panel display and semiconductor manufacturing. Removal of KrF PR, i-line PR, and negative color filter PR layers on a 6th-generation large area (1640 × 30 mm²) substrate was investigated by making use of a dielectric barrier discharge (DBD) plasma device, which is with a large number of gas-flowing holes. The nitrogen DBD plasma was generated with a mixture of compressed dry air (CDA) and SF₆. To prevent thermal shrinkage of the PR layer, samples were maintained at a temperature less than 100 °C. Uniformity and reproducibility experiments have been carried out in terms of treatment time. Eventually, we obtained an ashing rate of about 600 nm/min for negative color filter PR, and 450 nm/min for KrF and i-line PR at a CDA concentration of 1%, a SF₆ concentration of 0.5%, a carrier N₂ gas flow rate of 1500 liters per minute (lpm) and at an applied power of 8 kW. Amorphous-Si layer loss which strongly depends on the fluorine radicals was at an acceptable level of 5 nm/min in the given conditions.     

Anisotropic etching of silicon in SF6 plasma

The reactive ion etching of silicon in SF₆ plasma is considered. During the experiment, silicon substrates are etched in SF₆ plasma at different pressures and energies of incident ions. High etching anisotropy is achieved decreasing the pressure in the reactor and increasing the energy of the bombarding ions. The obtained experimental measurements are compared with theoretical calculations. It is determined that the temperature of the sidewalls decreases with the decrease of concentration of F atoms due to suppressed plasmochemical etching of silicon. The etching anisotropy increases with the decrease of concentration of F atoms due to decreased desorption of SiF₄ molecules.     

Inversion of the electron energy distribution in hollow-cathode glow discharge in nitrogen-sulfur hexafluoride gas mixture

The electron energy distribution function (EDF) in a hollow-cathode glow discharge in nitrogen-sulfur hexafluoride (N₂-SF₆) gas mixture has been studied by experimental and theoretical methods. On adding a small amount of SF₆ to N₂, the density of electrons in the energy interval (2−4 eV) of the EDF inversion increases by approximately one order of magnitude.     

Impact of SF6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF₆ gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl₄ plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF₆ plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.     

Studying transverse volume discharge in a helium-propane mixture

The electrical properties and emission characteristics of plasma formed in a transverse volume discharge in helium-propane (He-C₆H₁₄) mixtures at a total pressure of 3–15 kPa have been studied. The spatial parameters, plasma emission spectra, and current and voltage oscillograms of the discharge, as well as the yield of carbon-containing products of propane decomposition at various pressures and compositions of the He-C₆H₁₄ gas mixture, have been determined.     

The influence of metal vapors resulting from electrode evaporation in a thermal puffer-type circuit breaker

The present work is a computational investigation of thermal plasmas in SF₆ switchgear, which burns in a mixture of working gas and PTFE and metal vapors resulting from nozzle ablation and electrode erosion, respectively. In order to solve the concentration of PTFE and electrode vapors conservation equations for the PTFE vapor and the metal vapor are solved together with the governing equations for mass, momentum, and energy of the gas mixture under the assumptions of local thermodynamic equilibrium (LTE) and local chemical equilibrium (LCE). The influence of metal vapor due to electrode evaporation on the arc behavior in SF₆ switchgear can be explained with some important parameters indicating the characteristics of SF₆ arc plasma. First, temperature in arc column is considerably different from that of the case without contact evaporation. The contamination by metal vapors broadens the arc radius and lowers the core temperature when compared to the case without evaporation. Second, the concentration of PTFE vapor from nozzle surfaces can more easily move to the expansion volume during high current period due to the existence of metal vapors near electrodes. Finally, the electrical conductivity in the arcing zone can be higher and may negatively affect the interruption limit of switchgears due to the increasing rate of breakdown by the metal vapors between the electrodes.     

Surface acoustic wave excitation on SF6 plasma-treated AlGaN/GaN heterostructure

In this work, we introduce a new modified approach to the formation of interdigital transducer (IDT) structures on an AlGaN/GaN heterostructure. The approach is based on a shallow recess-gate plasma etching of the AlGaN barrier layer in combination with “in-situ” SF₆ surface plasma treatment applied selectively under the Schottky gate fingers of IDTs. It enables one to modify the two-dimensional electron gas (2DEG) density and the surface field distribution in the region of the IDTs, as is needed for the excitation of a surface acoustic wave (SAW). The measured transfer characteristics of the plasma-treated SAW structures revealed the excitation of SAW at zero bias voltage due to fully depleted 2DEG in the region of the IDTs. High external bias voltages are not necessary for SAW excitation. SIMS depth distribution profiles of F atoms were measured to discuss the impact of SF₆ plasma treatment on the performance of the AlGaN/GaN-based IDTs.     

Simulations of Si and SiO2 etching in SF6 + O2 plasma

Chemical etching of Si and SiO₂ in SF₆ + O₂ plasma is considered. The concentrations of plasma components are calculated using values extrapolated from experimental data. Resulting calculations of plasma components are used for the calculation of Si and SiO₂ etching rates. It is found that the reaction constants for reactions of F atoms with Si atoms and SiO₂ molecules are equal to (3.5 ± 0.1) × 10⁻² and (3.0 ± 0.1) × 10⁻⁴, respectively. The influence of O₂ addition to SF₆ plasma on the etching rate of Si is quantified.     

Study for amorphous silicon etching process using dielectric barrier discharge

Characteristics of amorphous silicon (a-Si) etching using atmospheric pressure plasma discharge had been studied. Dielectric barrier discharge (DBD) plasma with nitrogen gas was employed for the study. The active chemical agent for etching was generated by mixing a small quantity of sulfur hexafluoride (SF₆) gas into the plasma. The two distinguishable plasma zones are generated with the specially designed DBD plasma generator. The one is the main discharge zone generated between the two parallel plate electrodes. And the other one is downstream plasma zone extracted from the main discharge zone through the holes perforated on the bottom electrode. A test specimen was etched located at the plasma zone and moved the zone several times for etching on a temperature controlled stage. The etch rate of a-Si and the selectivity to silicon nitride (SiNx) were improved by addition of hydrogen (H₂) or methane (CH₄) gas into the plasma. However, when the specimen temperature was lower than 100 °C with H₂ or CH₄ gas added plasma condition, a-Si layer was not etched at all, but in the range of 100-140 °C of specimen temperature, the a-Si layer started to be etched while the influence of the specimen temperature on etching of a-Si was ignorable in that temperature range. At the optimized condition, the a-Si etch rate was up to 3000 A/min in the downstream plasma zone with the 3 mm of the distance between the surface of the specimen and the bottom side of the DBD plasma generator module. And the etch rate ratio between a-Si and SiNx was more than 100:1.     

Self-consistent particle simulation of radio frequency SF6 discharges: Role of ion recombination

The role of positive−negative ion recombination in simulating radio frequency (rf) SF₆ discharges is investigated based on Nanbu and Denpoh's theory [J. Phys. Soc. Jpn. 1998; 67: 1288]. The model of ion recombination is combined with the Particle-in-Cell/Monte Carlo (PIC/MC) simulation. Results show that the ion recombination is a dominant factor in simulating a steady rf discharge of SF₆. A criterion determining whether a periodic steady state is reached is proposed based on the ion recombination rate. It is found that the ion recombination rapidly increases as the gas pressure increases. From 25 to 50 m Torr, the loss of positive ions caused by ion recombination becomes comparable with that lost from collisions with electrodes.     

Use of plasma impedance monitoring for the determination of SF6 reactive ion etch process end points in a SiO2/Si system

It is shown here that plasma impedance monitoring can be used successfully to determine the end point of reactive ion etching of a SiO₂ layer lying on a Si substrate in SF₆ plasma. The usefulness of this technique is demonstrated using a commercial Plasma Impedance Monitoring (PIM) system. The end point conditions are tested by monitoring changes in the fundamental and the first four harmonic components of the RF current, RF voltage, phase between RF voltage and current, RF discharge power and RF impedance. The best process monitoring parameter found in this work is modeled as a polynomial equation of RF input power, chamber pressure and gas flow rate, from which the end point can be predicted with good precision and easily detected by the PIM. The end point conditions are confirmed by both Fourier Transform Infrared Spectroscopy (FTIR) measurements and via observation of plasma color changes. Received:15 June 2001 / Accepted: 18 June 2001