Neutral Loop Discharge Plasma Research Articles

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

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.

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.

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.

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.

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.

Observation of Characteristics of Magnetic Neutral Loop Discharge Plasma Appearing at Antenna in RF Circuit

The characteristics of magnetic neutral loop discharge (NLD) plasma have been studied by measurement of a high voltage appearing at an antenna in RF circuit. Peak-to-peak voltage Vpp at the antenna coil supplying power into plasma varied depending on the configuration and strength of the magnetic field. By analysis of the measured Vpp using the equivalent circuit model, it is deduced quantitatively that the power deposited into plasma in NLD is larger than that deposited in inductively coupled plasma (ICP).

Submicron dry-etching behavior of β-FeSi 2 thin films towards fabrication of photonic crystals

We have investigated dry-etching properties of polycrystalline β-FeSi 2 films formed by ion-beam sputter-deposition (IBSD) and the films epitaxially grown on Si by metal organic chemical vapor deposition (MOCVD) in order to realize fabrication of photonic crystals with several hundreds nanometers in dimension. Using reactive ion etching with magnetic neutral loop discharge (NLD) plasma, we succeeded in realizing a comparatively higher etching rate than that obtained by inductively coupled plasma (ICP). Both reactive ion etching and impact ion etching modes may contribute to etching of β-FeSi 2. We have fabricated a two-dimensional photonic crystal of β-FeSi 2 on Si substrates and confirmed its predicted photonic properties in a reflectance spectrum of polarized light.

WITHDRAWN: CVD application of a neutral loop discharge plasma for carbon nanotubes synthesis

This article has been withdrawn consistent with Elsevier Policy on Article Withdrawal ( http://www.elsevier.com/locate/withdrawalpolicy ). The Publisher apologizes for any inconvenience this may cause.

Surface roughness of Ti:LiNbO3 etched by Ar∕C3F8 plasma and annealing effect

We describe the reactive ion etching of Ti:LiNbO3 single crystal in gas mixtures containing C3F8 and Ar using neutral loop discharge plasma. The surface roughness of etched Ti:LiNbO3 surface under various gas mixture conditions was analyzed by using atomic force microscopy and scanning electron microscopy. The annealing effect of an etched ridge structure waveguides was also observed.

Asymmetry ridge structure fabrication and reactive ion etching of LiNbO3

Abstract Ti-indiffused LiNbO3 waveguides have been used for various high speed optical devices, such as modulators, switches and sensors, because of their electro-optic effect. In order to broaden the modulation bandwidth of an optical modulator above 10 GHz, both impedance matching and optical and RF phase velocity matching should be obtained at the high frequency range. This can be obtained by increasing the effective contact area of the CPW (co-planar waveguide) electrodes with air by means of LiNbO3 substrate etching. We studied the properties of LiNbO3 dry etching, in terms of the etching rate, etching angle and surface roughness, using a neutral loop discharge (NLD) plasma.

초고속 광변조기 제작을 위한 LiNbO3도파로의 건식식각

Ti-indiffused LiNbO<TEX>$_3$</TEX>waveguide have been used to various high speed optical device based on electro-optic effect such as modulators, switches, and sensor, etc. In order to high speed modulation of optical modulator have, one of the further devices, needed to increasing of electrode surrounding air by LiNbO<TEX>$_3$</TEX>dry etching because of impedance matching for optical and RF phase velocity between waveguide and electrode. We studied property of LiNbO<TEX>$_3$</TEX>dry etching after waveguide patterning lot optical modulation by using neutral loop discharge (NLD) plasma.

Etching characteristics of porous silica (k=1.9) in neutral loop discharge plasma

Etching characteristics of a porous silica material (ISM-1.5™ produced in ULVAC, Inc.) were investigated and compared with those of thermal oxide. The etch rate of porous silica in magnetic neutral loop discharge plasma was approximately two times higher than that of thermal SiO2 film when linear saturated perfluorocarbon compounds were used. This may be due to the low film density of the porous silica. However, in the case of C4F8 (octafluorocyclobutane) plasma, the etch rate ratio to SiO2 was about 1.45. When C4F6 (CF2=CFCF=CF2: hexafluorobutadiene) was used, the etch rate ratio was also very low (0.6). So, the etch rate strongly depended on the gas structure, whereas the SiO2 etch rate did not depend on the gas species and was almost constant. Through mass spectrometry and x-ray photoelectron spectroscopy measurements, it was deduced that the fluorocarbon polymer formed in the pore suppressed the etch rate of porous silica in C4F8 or C4F6 plasmas.

Investigations of Surface Reactions in Neutral Loop Discharge Plasma for High-Aspect-Ratio SiO2 Etching

The relationship between fine etching and gas structure in magnetic neutral loop discharge (NLD) plasma has been investigated using C4F8, C3F8, and CF3FOC=CF2 (HFE-216) gases. It was found that CF3+ ions were effectively generated in the HFE-216 plasma compared with those in the C4F8 or C3F8 plasma under the same conditions. Hydrofluorocarbon (HFC) gases such as CH2F2 (HFC-32) and CH3CHF2 (HFC-152a) were also employed to realize highly selective etching for SiO2 to the photoresist. C1s X-ray photoelectron spectra showed a prevalence of C–C and C–CFx bonds in the films deposited on the surface in the HFC plasma. This implies that the deposited film was mainly composed of carbon atoms. It was also found from X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared (FTIR) analyses that the chemical state of fluorine in this film was not C–F, but H–C–F. This may lead to the realization of microloading free etching with high resist selectivity in the HFE-216/HFC-152a mixture plasma. Studies on the relationship between etch performance and thin H–C–F polymer film formation were carried out in the HFE-216/HFC-152a mixture plasma. It can be thought that the interaction of the H–C–F film on the surface and CxFy species from the plasma is very low compared with that of a C–F film. The H–C–F film on the sidewall may play serve as a lubricant and may transport etchants to the bottom of the etched pattern. As a result, 50-nm-diameter holes and 40 nm space patterns with aspect ratios of 18 and 22.5, respectively, were successfully fabricated in the HFE-216 + HFC-152a + O2 plasma. The H–C–F film formation on the etched surface enables high-aspect-ratio etching with high selectivity.

Organic Low-k Dielectric Material Etching by CH3NH2/N2 Plasma

An etching process has been developed for an organic low-k dielectric material, FLARE, that uses a gas chemistry of CH3NH2/N2 in a magnetic neutral loop discharge (NLD) plasma system. The organic low-k etching can provide a normal-taper profile with no micro-trenching and little hard-mask erosion, which are necessary for fabricating Cu/organic low-k damascene multilevel interconnects. Inverse reactive ion etching (RIE) lag was observed under conditions that produced the normal-taper profile. We evaluated the amount of ions and radicals generated in CH3NH2/N2, CH4/N2, and H2/N2 plasmas by means of quadrupole mass spectrometry (QMS) and analyzed films deposited on a bare Si substrate in CH3NH2/N2, CH4/N2, and H2/N2 plasmas by using X-ray photoelectron spectroscopy (XPS). We found that a normal-taper etching profile was obtained when the C/N concentration ratio of the deposited film was about 2–3 in the CH3NH2/N2 plasma.