Neutral Loop Discharge Research Articles

Foundations of magnetized radio-frequency discharges

This is the second part of a set of two papers on radio-frequency (RF) discharges, part of a larger series on the foundations of plasma and discharge physics. In the first paper (Chabert et al 2021 Plasma Sources Sci. Technol. 30 024001) the two basic configurations of RF discharges commonly used in industrial applications, the capacitive and the inductive discharges, are presented. The introduction of an external magnetic field to these discharges results in not only a quantitative enhancement of their capabilities but also leads to qualitatively different interaction mechanisms between the RF field and the plasma. This provides rich opportunities for sustaining dense plasmas with high degrees of ionization. On one hand, the magnetic field influences significantly the particle and energy transport, thus providing new possibilities for control and adjustment of the plasma parameters and opening even lower operation pressure windows. On the other hand, when the magnetic field is introduced also in the region where the plasma interacts with the RF field, qualitatively new phenomena arise, that fundamentally change the mechanisms of power coupling to the plasma—the electromagnetic energy can be transported as waves deeper into the plasma volume and/or collisionlessly absorbed there by wave resonances. The characteristics of these discharges are then substantially different from the ones of the standard non-magnetized RF discharges. This paper introduces the physical phenomena needed for understanding these plasmas, as well as presents the discharge configurations most commonly used in applications and research. Firstly, the transport of particles and energy as well as the theory of waves in magnetized plasmas are briefly presented together with some applications for diagnostic purposes. Based on that the leading principles of RF heating in a magnetic field are introduced. The operation and the applications of various discharges using these principles (RF magnetron, helicon, electron cyclotron resonance and neutral loop discharges) are presented. The influence of a static magnetic field on standard capacitive and inductive discharges is also briefly presented and discussed.

A Study on the Optimized Ohmic Contact Process of AlGaN/GaN-Si MIS-HEMTs

AlGaN/GaN-Si based MIS-HEMTs are considered as the popular candidates for application in the 5G communication system due to their competitive characteristics and low cost. Ohmic contact, as an important fabrication process, significantly affects the performance of the device. In this study, the ohmic contact process, including the SiN passivation layer etching, surface treatment, and barrier layer etching, was studied in detail in order to effectively optimize the device performance. It is observed that the sample with the SiN passivation layer etched by the magnetic neutral loop discharge (NLD) resulted in a lower contact resistance as compared to the reaction ion etching (RIE). The sample surface treated with the O plasma and pickled in the HCl:H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O = 1:10 liquid could effectively remove the pollutants and oxides from the surface, thus, correspondingly presenting a lower ohmic contact resistance as compared to the N2 plasma. Meanwhile, an optimum etching depth was developed with the ICP process for 6 min with an etching speed of 1.6 nm/min. A contact resistance of 0.76 Ω · mm and square resistance of 274.63 ohm/sq were observed under the above-mentioned optimized ohmic contact process. The AlGaN/GaN-Si MIS-HEMT with gate length of 0.5 μm, gate-source space of 1 μm, gate-drain space of 2.5 μm, and gate width of 100 μm was fabricated using the optimized process. A saturation current density of 794.30 mA/mm and the maximum transconductance of 16.86 mS were observed. The findings in this study provide the experimental basis for the manufacturing of AlGaN/GaN-Si based MIS-HEMTs for RF applications.

Study of High Aspect Ratio NLD Plasma Etching and Postprocessing of Fused Silica and Borosilicate Glass

In this paper, we report magnetic neutral loop discharge (NLD) plasma etching of fused silica (FS) and borosilicate glass (BSG), demonstrating high aspect ratio deep etch (100 μm) with vertical walls (<;3° deviation from vertical). This paper for the first time presents the systematic study of FS and BSG deep etching in NLD plasma. Four different masking materials have been explored including metal, amorphous silicon, bonded silicon, and photoresist. Etch parameters were optimized to eliminate unwanted artifacts, such as micro-masking, trenching, and faceting, while retaining a high aspect ratio (up to 7:1 for FS and 8:1 for BSG). In addition, a method for sidewall roughness mitigation based on postfabrication annealing was developed, showing the sidewall roughness reduction from the average roughness (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) 900 to 85 nm. Further advances in deep plasma etching processes may enable the use of FS and BSG in the fabrication of precision inertial MEMS, micro-fluidic, and micro-optical devices.

Effect of Substrate Bias on Production and Transport of Etchant Ions in a Magnetic Neutral Loop Discharge Plasma

Production and transport of etchant ions in an RF (13.56 MHz) magnetic neutral loop (NL) discharge plasma in the presence and the absence of an ac (1.695 MHz) substrate bias were simulated using a Monte Carlo method. Under the bias, electrons accelerated near the NL were attracted to the substrate and produced more etchant ions near the substrate than in the absence of the bias. Effect of bias leading to enhancement of the ion inflow to the substrate was discussed.

Control of Magnetic Field for Sustainment of Ion Production and Uniform Ion Flux to Substrate in Neutral Loop Discharge Plasma

We simulated electron and ion motions in a neutral loop discharge plasma under the control of the foot of the separatrix sweeping over the substrate and the neutral loop moving within a short distance from the RF antenna by a Monte Carlo method. We analyzed the distributions of ion production and ion flux to the substrate. We found that ion production is sensitive to the magnetic field gradient rather than to the electric field strength. Moreover, by superposing the flux distributions weighted by the residence time of the foot of the separatrix on the substrate, we obtained a uniform time-averaged distribution of the ion flux to the substrate in a radius range of r = 4.0 to 14.0 cm with σ/m = 0.25% (m: mean, σ: standard deviation). © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 185(1): 9–16, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22417

Characteristics of Electron Conduction in a Gas under a Quadrupole Magnetic Field and Radio-Frequency Electric Fields

Electron conduction in CF4 under a simplified model quadrupole magnetic field (QMF) and rf electric fields at f = 13.56, 27.12, and 40.68 MHz was analyzed using a Monte Carlo method, following the manner of electron swarm analyses. This field configuration simulated current paths of magnetic neutral loop discharge plasmas used for dry etching. The electron conduction therein was focused on as an elemental process of plasma power deposition. The electrons formed a star-shaped distribution with four rays along the separatrices of the QMF. The effective electron conductivity quantified as the amplitude of mean electron velocity was high around the central axis. The time-averaged mean electron velocities in the four regions of the QMF were biased under a rectification effect of the magnetic field. With increasing f, the high-conductivity region expanded and ionization was promoted. An enhancement of plasma power deposition under high driving frequencies was indicated.

Effects of Micromachining Processes on Electro-Osmotic Flow Mobility of Glass Surfaces

Silica glass is frequently used as a device material for micro/nano fluidic devices due to its excellent properties, such as transparency and chemical resistance. Wet etching by hydrofluoric acid and dry etching by neutral loop discharge (NLD) plasma etching are currently used to micromachine glass to form micro/nano fluidic channels. Electro-osmotic flow (EOF) is one of the most effective methods to drive liquids into the channels. EOF mobility is affected by a property of the micromachined glass surfaces, which includes surface roughness that is determined by the manufacturing processes. In this paper, we investigate the effect of micromaching processes on the glass surface topography and the EOF mobility. We prepared glass surfaces by either wet etching or by NLD plasma etching, investigated the surface topography using atomic force microscopy, and attempted to correlate it with EOF generated in the micro-channels of the machined glass. Experiments revealed that the EOF mobility strongly depends on the surface roughness, and therefore upon the fabrication process used. A particularly strong dependency was observed when the surface roughness was on the order of the electric double layer thickness or below. We believe that the correlation described in this paper can be of great help in the design of micro/nano fluidic devices.

Characterization of neutral loop discharges in the VINETA device

A neutral loop (NL) discharge has been implemented in the linear plasma device VINETA with three different antenna configurations. Radial profile measurements of the electron density, temperature and floating potential are performed at various neutral loop radii and compared to single particle simulations. Local density maxima are observed near the NL, for which several mechanisms have been proposed. The measured temperature profiles do not support the model of a thermal heating process, whereas the floating potential profiles corroborate the assumption of a locally enhanced induction current.

Control of Magnetic Field for Sustainment of Ion Production and Uniform Ion Flux to Substrate in Neutral Loop Discharge Plasma

We simulated the electron and ion motions in a neutral loop discharge plasma under the control of the foot of separatrix sweeping over a substrate and the neutral loop moving within a short distance from the RF antenna by a Monte Carlo method. We analyzed the distributions of ion production and ion flux to the substrate. We revealed that ion production is sensitive to the gradient of magnetic field rather than the electric field strength. Moreover, by superposing the flux distributions weighted by the passage time of the foot of separatrix on the substrate, we obtained a uniform time-averaged distribution of ion flux to the substrate in a radius range of r = 4.0-14.0 cm with σ/m = 0.25% (m: the average, σ: the standard deviation).

Structure and dynamics of a magnetic neutral loop discharge plasma described using electron motion in a quadrupole magnetic field

Electron motion in a CF4 magnetic neutral loop discharge (NLD) plasma was simulated using a Monte Carlo method. The spatial distribution of electrons illustrated the fundamental structure of the NLD plasma, and its dynamics were depicted from the distributions of the mean electron energy, the electron energy gain and the azimuthal electron flux. The peak of mean electron energy appeared at the neutral loop (NL), which confirmed that electrons gain energy near this loop. High mean electron energy was observed not only near the NL but also along the separatrices of the quadrupole magnetic field. Energetic electrons were transported along the separatrices and induced ionization at those locations. However, the electron distribution had valleys along the separatrices, because electrons accelerated near the NL were likely to leave this region where the binding of the magnetic field is weak. The azimuthal electron flux representing the loop plasma current showed that the electron conduction path around the NL, which has conventionally been modelled as a ring conductor, has a particular directionality due to the rectification effect of antiparallel magnetic fields composing the quadrupole magnetic field. The directionality in the upper and lower regions of the quadrupole magnetic field was opposite to that in the inner and outer regions.

Control of Magnetic Field in Neutral Loop Discharge Plasma for Uniform Distribution of Ion Flux on Substrate

The production and transport of ions in a neutral loop (NL) discharge plasma under a quadrupole magnetic field are investigated using a Monte Carlo method. The ion production occurred around the separatrices of the quadrupole magnetic field and decreased as the NL shrunk. The ion flux on the substrate had its peak near the foot of a separatrix. It was demonstrated that the time-averaged ion flux on the substrate can be distributed uniformly by controlling the magnetic field so that the foot of the separatrix sweeps over the substrate.

Electron Distribution in a Quadrupole Magnetic Field to Drive Magnetic Neutral Loop Discharge Plasma

We simulated the electron motion in a quadrupole magnetic field that drives a neutral loop discharge (NLD) plasma using a Monte Carlo method. The structure of the electron distribution in the NLD plasma was observed. Electrons underwent reciprocating motion between reflecting boundaries as they gyrated along magnetic field lines. During this motion, they deviated from the neutral loop and concentrated beside the separatrices of the quadrupole magnetic field.

Rectification effect of gradient antiparallel magnetic fields on electron conduction in a gas under a radio-frequency electric field

Electron transport in CF4 under linearly gradient antiparallel magnetic fields was analysed in order to investigate the fundamental properties of magnetic neutral loop discharge plasmas used for material processing. The electron motion was simulated by a Monte Carlo method under a radio-frequency (rf) electric field applied perpendicularly to the directions of both the magnetic field and its gradient. Two typical electron motions, meandering in a weak magnetic field and gyration in a strong magnetic field, were observed with particular directionalities. The meandering electrons drifted forward on average similarly to those under a dc electric field. The gyration induced an electron drift towards the inverse direction. The direction of electron flux was dependent not only on the rf phase but also on the distance from the magnetically neutral midplane between the antiparallel magnetic fields. The electron conduction path formed along the midplane had a structure consisting of forward and inverse lanes. A peculiar result was that the direction of local electron flux was always forward in the strong magnetic field whereas the drift of gyrating electrons was towards the inverse direction. This seemingly paradoxical result can appear in the presence of the density gradient of the electron distribution.

Wave propagation and noncollisional heating in neutral loop and helicon discharges

Heating mechanisms in two types of magnetized low pressure rf (13.56 MHz) discharges are investigated: a helicon discharge and a neutral loop discharge. Radial B-dot probe measurements demonstrate that the neutral loop discharge is sustained by helicon waves as well. Axial B-dot probe measurements reveal standing wave and beat patterns depending on the dc magnetic field strength and plasma density. In modes showing a strong wave damping, the plasma refractive index attains values around 100, leading to electron-wave interactions. In strongly damped modes, the radial plasma density profiles are mainly determined by power absorption of the propagating helicon wave, whereas in weakly damped modes, inductive coupling dominates. Furthermore, an azimuthal diamagnetic drift is identified. Measurements of the helicon wave phase demonstrate that initial plane wave fronts are bent during their axial propagation due to the inhomogeneous density profile. A developed analytical standing wave model including Landau damping reproduces very well the damping of the axial helicon wave field. This comparison underlines the theory whereupon Landau damping of electrons traveling along the field lines at speeds close to the helicon phase velocity is the main damping mechanism in both discharges.

Iron silicide photonic crystals and light propagation property

We report fabrication of iron silicide photonic crystals and properties of light propagation in β-FeSi2 two-dimensional (2D) or slab-type photonic crystals (PhC) designed to have their photonic band gaps near 1.55 μm. β-FeSi2 films were etched by neutral loop discharge reactive ion etching (NLD-RIE) which used a common Si process gas (SF6 + CHF3) system. We propose a new idea to reduce the reflection loss at light incidence. The idea is a characteristic PhC structure (called a conjugated PhC structure) in which inversion photonic patterns are fabricated at the incident interface and they have common photonic band-gap against the wavelength of propagating light. We confirmed large reduction of the reflection loss at the conjugated photonic structure.

A New High-Performance Ion Source Based on Magnetically Neutral Loop Discharge

We have developed a new high-performance ion source on the basis of the high-density plasma formation method known as magnetically neutral loop discharge (NLD). The ion source consists of three separate electromagnetic coils, an ion extraction electrode, and a quartz vessel plasma chamber with a one-turn RF antenna coil, which is the main component of conventional RF ion sources. The three electromagnetic coils are located around the periphery of the plasma chamber. The current in the middle coil flows opposite to the currents in the top and bottom coils. With this configuration, our source produces a high plasma density of 1011 cm-3, which is about ten times higher than that of a conventional source under a low gas pressure of 0.1 Pa. In addition, it is possible to control both the ring like plasma diameter and the high-density plasma generation position in the plasma chamber. As a result, we achieved a high ion current density of up to 1 mA/cm2 at 50 V of ion extraction voltage, which is almost five times higher than the conventional source. Furthermore, we also obtained a high ion current uniformity of ±5% over a 6-in.-diameter wafer using the source's plasma space controllability. A processing system that uses this ion source will contribute to faster processing, high-quality processing, and excellent-uniformity processing for ion beam etching, ion beam deposition, and ion beam sputter deposition. Many other applications are also expected.

Numerical Simulation of Electron Transport in Electric and Magnetic Fields for Analysis of Electron Temperature and Number Density Profiles Measured in Argon Magnetic Neutral Loop Discharge Plasma

A Monte Carlo simulation of electron transport in electric and magnetic fields was performed to analyze experimental data of the electron temperature Te and electron number density ne measured in a magnetic neutral loop discharge (NLD) plasma driven in Ar at 0.13 Pa. Te and ne in the vicinity of the substrate holder were measured with a triple probe, and their radial profiles had peaks at different radial positions. The simulation reproduced these peak positions well under the chosen boundary condition that the electron reflectivity of the side wall was lower than that of the reactor ceiling and the substrate holder. It was explained that the Te peak was formed by high-energy electrons transported from the neutral loop along a separatrix of the quadrupole magnetic field and that the ne peak consisted of electrons undergoing reciprocating motion between the reactor ceiling and the substrate.

Etching Technologies in NLD (magnetic Neutral Loop Discharge) Plasma

NLD (magnetic Neutral Loop Discharge) plasma has two major characteristic features that high density one is generated at the lower pressure than 1 Pa and is controllable by changing the magnetic coil current. Utilizing this feature, ionic etching should be carried out at the lower pressure than 1 Pa for chemical reactive substrates, for example, organic materials or ArF photo resists, because ionic etching is low selective and low reactive. In Si etching process, the NLD plasma is utilized by employing sputter/etching method, which is scheduled cyclic. The NLD plasma is very stable for abrupt changing of the process pressure. This is brought on by weakly magnetized plasma. When PTFE (Poly Tetra-Fluoro-Ethylene) is used as a sputter target, deep etching of 180 μm is achieved for 7 μm pattern with aspect ratio of 25.7.

A Novel TSV Etching using NLD and VHF CCP Plasma For 3-D Stacked Devices.

In this study, the etch development of high etch rate and high aspect ratio for the fabrication of various diameters of thru silicon via is described. There are a lot of applications about TSV's methods. There TSV need to meet strict requirements with respect to high etch rate, sidewall roughness and undercut. For diameter of vias down to 10um a SiO2 hard mask based SF6/O2 etch approach is used by magnetic neutral loop discharge plasma (NLD plasma). This system can be used for oxide etching and Si etching. In the case of oxide etching, some magnetic coil currents are applied to generate magnetic neutral loop in the vacuum chamber in order to high density plasma in the low pressure below 1 Pa. On the other hand, in the case of Si etching, the pressure is raised up to 2∼10 Pa. As a result, an etched profile of TSV with diameter of 1.5 um and the etched depth of 11.5 um, aspect ratio is 7.6, was achieved. The etch rate is about 4.5 um/min. For diameter of vias up to 10um, a resist mask based and SF6/O2 etch approach is also used by very high frequency coactivity coupled plasma. VHF CCP etching system was used to high-pressure process, more than 50Pa, as a purpose to high etch rate of up to 10um pattern's TSV etching. As a result, a vertical profile of TSV with diameter of 50 um and the etched depth of 50 um, aspect ratio is 1.0, was achieved. The etch rate is about 45 um/min. NLD and VHF CCP processes are non-cycle etch methods, and to successfully smooth pattern TSV.

Collisionless wave damping in neutral loop discharges

Heating mechanisms in a low pressure magnetized argon radio-frequency neutral loop discharge (NLD) are investigated. Results obtained in the NLD are compared with results obtained in an azimuthally isotropic m = 0 helicon discharge operated in the same setup. B-dot probe measurements are carried out showing that helicon waves are excited in the NLD. Phase resolved optical emission spectroscopic measurements demonstrate that electrons oscillate effectively not in the neutral loop (NL) but in a toroidal cusp located above the NL. In a simple transformer model we consider a diamagnetic current induced from the antenna windings into this cusp. The resulting calculated fields reproduce qualitatively well the measured helicon-NLD field. Furthermore, standing wave and beat patterns emerge in the axial direction. Depending on the dc magnetic field strength and plasma density different damping of the axial wave takes place. At strong damping, measured and calculated wave phase velocities approach the electron thermal velocity. Observed optical emission patterns corroborate the hypothesis of electron Landau damping being the main heating mechanism in both discharges.