Low Electron Temperature Plasma Research Articles

Abnormal behavior of the plasma potential in an inductively coupled plasma with a DC-biased grid

Abstract The formation of the plasma potential and the generation mechanism of very low electron temperature plasma are investigated in an inductively coupled plasma with a DC biased grid. The electron temperature is controlled from 2.4 eV to 0.2 eV according to the grid voltage (10 V to −40 V). Interestingly, when the grid voltage is negatively biased, the electron temperature decreases and the plasma potential decreases with the grid voltage, but then increases below −10 V which is abnormal. This behavior of the plasma potential is abnormal since the plasma potential is generally proportional to the electron temperature. The main reason for the abnormal increase of the plasma potential is the difference in the flux of electrons and ions below the grid. As the grid is negatively biased, the electron flux is greatly reduced compared to the ion flux, leading to an increase in plasma potential. After −20 V, the plasma potential saturates, because although the number of electrons entering the grid decreases, the electron flux is maintained by secondary electrons generated in the grid mesh. This abnormal increase in plasma potential decreases with pressure. An increase in plasma potential with gas species is also observed. The same behavior is observed for Ar, He, and N2 gases. The abnormal behavior of the plasma potential is explained with the current continuity.

Is Westward Travelling Surge Driven by the Polar Cap Flow Channels?

AbstractFollowing substorm auroral onset, the active aurora region usually expands poleward toward the poleward auroral boundary. Such poleward expansion is often associated with a bulge region that expands westward and forms the westward travelling surge. In this study, we show all‐sky imager and Poker Flat Advanced Modular Incoherent Scatter Radar observations of two surge events to investigate the relationship between the surge and ionospheric flows that likely have polar cap origin. For both events, we observe auroral streamers, with an adjacent flow channel consisting of decreased density and low electron temperature plasma flowing equatorward. This flow channel appears to impinge and lead/feed surge formation, and to stay connected to the surge as it moves westward. Also, for both events, streamer observations indicate that, following initial surge development, similar flows led to explosive surge enhancements. The observation that the streamers are connected to the auroral polar boundary and that the flow channels consisted of low density, low electron temperature plasma suggests the possibility that the impinging plasma came from the polar cap. For both events, the altitude variations of F region plasma within the surges are related with aurora emission and the poleward/equatorward flow, and the surges develop strong auroral streamers that initiate along the poleward auroral boundary when contacted with the flow. These results suggest that the flow of polar cap origin, which maps to underlying processes in the magnetotail, may play a crucial role in auroral surges by feeding low entropy plasma into surge initiation and development, and also playing an important role in the dynamics within a surge.

Sheath characteristics in a magnetically filtered low density low temperature multicomponent plasma with negative ions

Negative ion rich multicomponent plasma is produced in a novel plasma device for sheath studies. The experimental volume is composed of low electron temperature ( T e ∼ 0.2 − 0.4 eV ) and low electron density ( n e ∼ 10 5 − 10 6 cm − 3 ) argon (Ar) plasma with negative ions. Negative ions are introduced in the argon plasma by injecting sulfur hexafluoride ( SF 6 ) gas. Plasma parameters in the device are tunable, very close to the ionospheric plasma conditions specific to the D-region. A magnetic filter is used, which allows selective diffusion of low energy electrons and ions from a DC discharge source. Positive ion and electron/negative ion sheaths formed in front of a biased metal plate are investigated using an emissive probe. Both the positive ion and electron/negative ion sheaths are found to be of similar thickness and symmetric structure. With an enhanced negative ion concentration, the sheath thickness is found to decrease. Poisson's equation is derived considering the electrons as Maxwellian. The experimental results closely follow the prediction of Poisson's equation and the Debye sheath model. Sheath studies as shown here, for laboratory plasmas with near-ionospheric parameters, are important for the understanding of spacecraft-to-plasma interactions and for the design of ionospheric instrumentation.

An electron Bernstein wave heating scheme for the Proto-MPEX linear device

Linear plasma devices, such as the Prototype Material Plasma Exposure eXperiment (Proto-MPEX), are an economic method to study plasma-material interactions under high heat and particle fluxes. The Proto-MPEX device at Oak Ridge National Laboratory is a high-density helicon plasma generator with additional resonant electron heating to study plasma-material interactions in ITER-like conditions. Here, we present modeling results of an electron Bernstein wave (EBW) heating scheme for this device using a microwave-based system. A 28 GHz injection system is available to provide power levels up to 100 kW injected into low electron temperature and high electron density plasmas. A modified version of the GENRAY ray-tracing code, GENRAY-C, has been used to determine the EBW and electron cyclotron heating wave accessibility for these overdense plasmas. Modeling has shown that greater than 80% of mode conversion from the ordinary mode to the EBW is possible for 28 GHz injection in Proto-MPEX. Calculations show that by utilizing the effects of Doppler-shifted resonance absorption, power deposition near the core is possible. Significant collisional damping, leading to edge absorption, is expected to occur for neutral pressures >1.0 mTorr; however, experiments can be designed to minimize these effects.

Model of a radio-frequency low electron temperature plasma source

The time-averaged plasma potential in a partially ionized plasma is directly linked to the electron temperature (Te). Plasmas with low Te or plasma potential are attractive for applications that require low ion energy at surfaces. This article describes a radio-frequency (RF) driven low Te plasma source. This source utilizes two RF supplies at 2 and 60 MHz to achieve the optimum plasma density and ion energy. The plasma source is divided into two regions separated by a perforated plate, which is metallic and grounded. A high density (>1017 m−3) plasma is generated in the primary region next to the RF powered electrode. The DC bias on the powered electrode is high (∼2000 V) leading to energetic ion bombardment on it. These ions produce secondary electrons that, under the low pressure condition under consideration, enter the plasma as a beam of energetic electrons and many of them reach the perforated plate. It is demonstrated that the slits in the perforated plate can be designed to prevent the RF primary plasma from leaking into the secondary region while still allowing the beam electrons to pass through. The plasma produced by the beam electrons has moderate density (∼1016 m−3) and Te < 0.5 eV. The influence of slit dimensions on the characteristics of the plasma in the secondary discharge region is examined in this article.

Ion and electron sheath characteristics in a low density and low temperature plasma

Ion and electron sheath characteristics in a low electron temperature (Te ∼ 0.25–0.40 eV) and density (ne ∼ 106–107 cm−3) plasma are described. The plasma is produced in the experimental volume through diffusion from a hot cathode discharge plasma source by using a magnetic filter. The electron energy distribution function in the experimental plasma volume is measured to be a narrow Maxwellian distribution indicating the absence of primary and energetic electrons which are decoupled in the source side by the cusp magnetic field near the filter. An emissive probe is used to measure the sheath potential profiles in front of a metal plate biased negative and positive with respect to the plasma potential. For a positive plate bias, the electron density decreases considerably and the electron sheath expands with a longer presheath region compared to the ion sheath. The sheath potential structures are found to follow the Debye sheath model.

Etching with atomic precision by using low electron temperature plasma

There has been a steady increase in sub-nm precision requirement for many critical plasma etching processes in the semiconductor industry. In addition to high selectivity and low controllable etch rate, an important requirement of atomic precision etch processes is no (or minimal) damage to the remaining material surface. It has traditionally not been possible to avoid damage in conventional radio-frequency (RF) plasma processing systems, even during layer-by-layer or ‘atomic layer’ etch. To meet these increasingly stringent requirements, it is necessary to have an accurate control over ion energy and ion/radical composition during plasma processing. In this work, a new plasma etch system designed to facilitate atomic precision plasma processing is presented. An electron sheet beam parallel to the substrate surface is used to produce a plasma in this system. This plasma has a significantly lower electron temperature Te ~ 0.3 eV and ion energy Ei < 3 eV (without applied bias) compared to inductively and capacitively coupled RF plasmas. Electron beam plasmas also have a higher ion–to–radical ratio compared to RF plasmas, so this plasma etch system employs an independent radical source for accurate control over relative ion and radical concentrations. A low frequency RF bias capability that allows control of ion energy in the 2–50 eV range is another important component of this plasma etch system. The results of etching of a variety of materials and structures in this low-electron temperature plasma system are presented in this study: (1) layer-by-layer etching of p-Si at Ei ~ 25–50 eV using electrical and gas cycling is demonstrated; (2) continuous etching of epi-grown µ-Si in Cl2-based plasmas is performed, showing that surface damage can be minimized by keeping Ei < 10 eV. Also presented are the results of molecular dynamics modeling of atomic precision etching at low Ei.

Effect of plasma parameters on characteristics of silicon nitride film deposited by single and dual frequency plasma enhanced chemical vapor deposition

This work investigates the deposition of hydrogenated amorphous silicon nitride films using various low-temperature plasmas. Utilizing radio-frequency (RF, 13.56 MHz) and ultra-high frequency (UHF, 320 MHz) powers, different plasma enhanced chemical vapor deposition processes are conducted in the mixture of reactive N2/NH3/SiH4 gases. The processes are extensively characterized using different plasma diagnostic tools to study their plasma and radical generation capabilities. A typical transition of the electron energy distribution function from single- to bi-Maxwellian type is achieved by combining RF and ultra-high powers. Data analysis revealed that the RF/UHF dual frequency power enhances the plasma surface heating and produces hot electron population with relatively low electron temperature and high plasma density. Using various film analysis methods, we have investigated the role of plasma parameters on the compositional, structural, and optical properties of the deposited films to optimize the process conditions. The presented results show that the dual frequency power is effective for enhancing dissociation and ionization of neutrals, which in turn helps in enabling high deposition rate and improving film properties.

Simulation of Z-pinch Al plasma radiation and correction with considering superposition effect

In this article, the X ray special line radiation from Z-pinch Al plasma is calculated by using non-local-thermodynamic-equilibrium model (NLTE) based on digital energy level. The electron density of Z-pinch plasma at centre is obtained. The single plasma state parameter cannot well describe the special line radiation. In the next step, the Z-pinch plasma is divided into two parts: high electron temperature plasma at centre and low electron temperature plasma. The superposition of radiations from two parts is analyzed, and modified calculation result is presented. The calculation results are significantly improved after the superposition of radiations from two parts has been taken into account.

Production of Low Electron Temperature Plasma and Coating of Carbon-Related Water-Repellent Films on Plastic Plate

Low electron temperature plasma with a value less than 2 eV was produced by the inductively coupled discharge with a three-turn spiral antenna and the radio-frequency-biased grid for hard coating of carbon-related water-repellent thin films on plastic plate. It was found that the electron temperature decreased with increasing a negative biasing voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> <; -100 V and, then, electron population becomes bicomponent with the higher temperature of about 10 eV and the lower temperature of less than 2 eV. The water-repellent thin films were also prepared on polycarbonate and silicon wafer substrates in the low electron temperature plasma using C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Ar mixture gases. The wetting contact angle and the pencil hardness were discussed.

Mode transition in magnetic pole enhanced inductively coupled argon plasmas

The electrical probe (Langmuir probe) diagnostics of different plasma parameters and operation regimes (E/H modes) of magnetic pole enhanced, inductively coupled (MaPE-ICP) argon plasmas are investigated. It is shown that uniform, high density (ne ∼ 1012 cm-3) and low electron temperature (Te ∼ 1.5 eV) plasma can be produced in low pressure argon discharges at a low power (100 W). It is found that an MaPE-ICP reactor operates in two different modes; capacitive (E mode) and inductive (H mode). No density jump or hysteresis are reported between these modes. The effect of pressure on transition power, where the mode changes from E to H mode at 20 sccm gas flow rate are studied and it is found that for all pressures tested (∼7.5 mTorr to 75 mTorr) the transition power remains same. In the inductive mode, the above plasma parameters show a smooth variation with increasing filling gas pressure at fixed power. The intensity of the emission line at 750.4 nm due to 2p1 → 1s2 (Paschen’s notation) transition, closely follows the variation of ne with RF power and filling gas pressure. Measured electron energy probability function (EEPF) shows that electron occupation mostly changes in the high-energy tail, which enlightens close similarity of the 750.4 nm argon line to electron number density (ne). The behaviour of the electron energy probability function (EEPF) with regard to pressure and RF power in two operational modes is presented.

Development of a 915 MHz ECR plasma source

We produced a 915 MHz ECR plasma with TM01 microwaves for different magnetic flux density distributions and examined the radial profiles of the ion saturation current as a function of pressure and power. It was found that the 915 MHz ECR plasma is uniform over 200 mm in diameter around the L-cutoff density and when a diverging magnetic field is used, a low electron temperature plasma is realized.

Charging of micrometre-sized dust grains in a low temperature and low density plasma produced using a magnetic filter

A magnetic filter plasma source has been designed to produce a low electron temperature (Te ∼ 0.15 eV) and low electron density (ne ∼ 106 cm−3) plasma volume. The parameters Te and ne are found to be controllable within a certain range by adjusting the discharge condition in the source. Charging of micrometre-sized dust grains in such a plasma has been investigated. The measured average charge on a dust grain (a few hundred electrons) is found to be very close to that observed in nature. When dust grains are allowed to enter perpendicularly into an external axial magnetic field applied to such a plasma, the average grain charge increases with increasing strength of the magnetic field (<150 G).

Rotation magnet sputtering: Damage-free novel magnetron sputtering using rotating helical magnet with very high target utilization

Novel magnetron-sputtering equipment, called rotation magnet sputtering (ROT-MS), was developed to overcome various disadvantages of current magnetron-sputtering equipment. Disadvantages include (1) very low target utilization of less than 20%, (2) difficulty in obtaining uniform deposition on the substrate, and (3) charge-up damages and ion-bombardment-induced damages resulting from very high electron temperature (&amp;gt;3eV) and that the substrate is set at the plasma excitation region. In ROT-MS, a number of moving high-density plasma loops are excited on the target surface by rotating helical magnets, resulting in very high target utilization with uniform target erosion and uniform deposition on the substrate. This excellent performance can be principally maintained even if equipment size increases for very large-substrate deposition. Because strong horizontal magnetic fields (&amp;gt;0.05T) are produced within a very limited region just at the target surface, very low electron-temperature plasmas (&amp;lt;2.5eV for Ar plasma and &amp;lt;1eV for direct-current-excited Xe plasma) are excited at the very limited region adjacent to the target surface with a combination of grounded plate closely mounted on the strong magnetic field region. Consequently, the authors can establish charge-up damage-free and ion-bombardment-induced damage-free processes. ROT-MS has been applied for thin-film formation of LaB6, which is well known as a stable, low-work-function bulk-crystal material for electron emissions. The work function of the LaB6 film decreased to 2.8eV due to enhanced (100)-orientation crystallinity and reduced resistivity realized by adjusting the flux of low-energy bombarding ions impinging on the depositing surface, which work very efficiently, improving the performance of the electron emission devices.

Fast Gas Replacement in Plasma Process Chamber by Improving Gas Flow Pattern

The precise and high-speed alteration of various gas species is important for realizing precise and well-controlled multiprocesses in a single plasma process chamber with high throughput. The gas replacement times in the replacement of N2 by Ar and that of H2 by Ar are measured in a microwave excited high-density and low electron-temperature plasma process chamber at various working pressures and gas flow rates, incorporating a new gas flow control system, which can avoid overshoot of the gas pressure in the chamber immediately after the valve operation, and a gradational lead screw booster pump, which can maintain excellent pumping capability for various gas species including lightweight gases such as H2 in a wide pressure region from 10-1 to 104 Pa. Furthermore, to control the gas flow pattern in the chamber, upper ceramic shower plates, which have thousands of very fine gas injection holes (numbers of 1200 and 2400) formed with optimized allocation on the plates, are adopted, while the conventional gas supply method in the microwave-excited plasma chamber uses many holes only opened at the sidewall of the chamber (gas ring). It has been confirmed that, in the replacement of N2 by Ar, a short replacement time of approximately 1 s in the cases of 133 and 13.3 Pa and approximately 3 s in the case of 4 Pa can be achieved when the upper shower plate has 2400 holes, while a replacement time longer than approximately 10 s is required for all pressure cases where the gas ring is used. In addition, thanks to the excellent pumping capability of the gradational lead screw booster pump for lightweight gases, it has also been confirmed that the replacement time of H2 by Ar is almost the same as that of N2 by Ar.

Low-Dielectric-Constant Nonporous Fluorocarbon Films for Interlayer Dielectric

Low-dielectric-constant (k<2.0) nonporous fluorocarbon films are formed using a new microwave-excited low electron temperature and high-density plasma system with a dual-shower-plate structure. In the new system, the material gas (C5F8) is supplied by a lower shower plate inserted in the diffusion plasma region of very low electron temperature (around 1–2 eV). An upper shower plate is used for supplying the plasma excitation gas in a uniform downflow in the chamber. Since such a gas flow pattern can prevent the penetration of the material gas into the plasma excitation region, the overdecomposition of the material gas can be markedly suppressed as compared with that in the case of using conventional plasma systems such as an electron cyclotron resonance (ECR) plasma source. As a result, fluorocarbon films can be formed by maximizing the original characteristics of material gases. The fluorocarbon films formed using the new system have not only low k but also low leakage current density, sufficient mechanical strength, strong adhesion, high heat resistance and good surface smoothness. Therefore, such films can be used in interlayer dielectrics in ultralarge-scale integration (ULSI) devices.

High-rate synthesis of microcrystalline silicon films using high-density SiH 4/H 2 microwave plasma

A high electron density (> 10 11 cm − 3 ) and low electron temperature (1–2 eV) plasma is produced by using a microwave plasma source utilizing a spoke antenna, and is applied for the high-rate synthesis of high quality microcrystalline silicon (μc-Si) films. A very fast deposition rate of ∼ 65 Å/s is achieved at a substrate temperature of 150 °C with a high Raman crystallinity and a low defect density of (1–2) × 10 16 cm − 3 . Optical emission spectroscopy measurements reveal that emission intensity of SiH and intensity ratio of H α/SiH are good monitors for film deposition rate and film crystallinity, respectively. A high flux of film deposition precursor and atomic hydrogen under a moderate substrate temperature condition is effective for the fast deposition of highly crystallized μc-Si films without creating additional defects as well as for the improvement of film homogeneity.

Development of a low temperature plasma source providing ion flow energy control

A low electron temperature plasma source suitable for control of ion flow energy along magnetic-field lines is developed, which is made up of an ion-production region and a plasma-synthesis region. A mesh-shaped electron emitter is installed in order to uniformly supply the electrons to the synthesis region. Our diagnoses using Langmuir probes and electrostatic ion energy analyzers prove that only the ion flow energy can be controlled under constant electron density and temperature. Moreover, our method is found to achieve a low electron temperature (Te ≃ 0.2–0.4 eV), yielding almost uniform radial and axial profiles of the averaged electron density, temperature, space potential and the ion flow energy.

GEOMETRY AND BIAS DEPENDENCE OF LOW-FREQUENCY RANDOM TELEGRAPH SIGNAL AND 1/f NOISE LEVELS IN MOSFETS

Low-frequency noise in MOSFETs is considered to originate from two distinctive sources: Random Telegraph Signal caused by carrier traps at the border of the SiO 2/ Si interface and 1/f fluctuation due to inherent nature of lattice scattering in a Si crystal. It is very important to distinguish these two mechanisms. Relative amplitude of RTS and 1/f noise depends on the number of carriers under the gate electrode, which makes it channel size as well as gate-bias dependent. In this paper, we discuss the dependence of the amplitudes of RTS and 1/f noise in MOSFETs on sample geometry and gate bias condition. We discuss low-frequency noise reduction by utilizing low electron-temperature plasma for gate oxidation as well.

Production of a large diameter ECR plasma with low electron temperature

A large diameter plasma over 300 mm in diameter is produced by electron cyclotron resonance (ECR) discharges using a cylindrical vacuum chamber of 400 mm in inner diameter. It is found that the plasma uniformity is improved by adding the nitrogen gas to pure Ar plasma. The electron temperature is decreased by adding the nitrogen gas. It is considered that the electron energy is absorbed in the vibrational energy of nitrogen molecules and the electron temperature decreases. Therefore, the adjunction of the nitrogen gas is considered to be effective for producing uniform and low electron temperature plasma.