Low-pressure High-density Plasma Research Articles

Growth of boron-doped diamond-like carbon films from a low-pressure high-density plasma in inductively coupled chemical vapour deposition

The main objective of this work was the preparation and optimization of boron-doped diamond-like carbon (DLC) films having a good crystalline structure, and electronic and optical properties, at an extremely low deposition pressure and relatively low growth temperature, by varying the flow rates of precursor gases (CH4, B2H6, and Ar) in the plasma controlled at 800 W of RF power. Planar inductively coupled plasma-enhanced chemical vapour deposition technique was used to enable deposition at ∼5.33 Pa and 450 °C, at a constant negative substrate bias of -40 V. The flow rate of the dopant gas (B2H6) was changed to obtain a set of samples, while the precursor gas (CH4) flow rate was kept fixed. The doped sample, prepared with B2H6/CH4 ratio (r) = 1 %, was found to possess a maximum ID/IG (intensity ratio of the D peak with the G peak) of ∼0.819 and a minimum IDia/IG ratio (relative intensity of the Diamond peak to the G peak) of ∼1.149. A high sp3 content of ∼50 % was revealed from the X-ray photoelectron spectroscopy analysis and the transmission electron microscopy images also indicated the presence of prominent 〈111〉, 〈220〉, and 〈311〉 crystallographic planes. The B-doped DLC film possessed enhanced electrical conductivity (σRT) of ∼3.02 ×10−5 S cm−1 relative to the intrinsic DLC films, a corresponding minimum of activation energy (ΔEH) (as calculated in the above room-temperature regime) of ∼170 meV, an optical band gap (Eg) of ∼3.35 eV, and the root mean square roughness of ∼48.12 nm.

Preparation of boron doped diamond-like carbon films in a low-pressure high-density plasma in RF ICP-CVD

The main objective of this work is to prepare electrically conducting boron doped diamond-like carbon (DLC) films with good crystalline qualities and high optical band gap in a low pressure and high-density plasma in an RF ICP CVD system. The main precursor gas used was CH4, with Ar being the diluent gas. B2H6 in He was used as the main source of boron, as the dopant, in the system. Planar inductively coupled technique enables the plasma to be ignited at a very low deposition pressure of 30 mTorr. A substrate temperature of 450 °C, a 900 W of RF power and a dc substrate bias of −40 V were used during the sample growth. By varying the B2H6/CH4 (=R) ratio, the optimized condition for preparing boron doped DLC thin films was obtained at R = 1. Raman analysis showed that this sample has the minimum ID/IG ratio ∼0.83, a maximum IDia/ID (∼1.18) and a maximum IDia/IG (∼0.98). The sp3 C-C content of 48.05% was calculated from XPS. A high optical band gap of ∼3.8 eV was obtained from the UV–Vis spectra. Boron doping was confirmed from the Raman, XPS and electrical conductivity measurements.

TALIF measurements of hydrogen and deuterium surface loss probabilities on quartz in low pressure high density plasmas

This article deals with surface loss on quartz of atomic hydrogen (H) and its isotope deuterium (D) in a low-pressure (10 Pa) pulsed inductively coupled plasma. The atomic temporal decay in the post discharge is measured by two-photon absorption laser-induced fluorescence (TALIF). From the loss rate, the atomic surface loss probability is determined. In pure hydrogen or pure deuterium gas, no isotopic effect on surface kinetics has been observed and the surface loss probabilities of H and D were found to be almost identical and equal to ∼1.8%. However, despite the lack of difference in surface loss probability, a net isotopic effect on surface loss rate due to the mass difference between the isotopes is measured. Hydrogen atoms diffuse faster and have higher flux to the plasma chamber walls than deuterium atoms. Hydrogen atoms are therefore lost at higher rate than deuterium atoms. Based on the observed isotopic difference and on the comparison between H and D TALIF signals, the isotopic effects on H and D atom production are discussed.

Microarcing-enhanced tungsten nano and micro-particles formation in low pressure high-density plasma

In this article, tungsten dust formation kinetics in two distinct plasma regimes (glow and micro-arc-assisted glow discharge) are compared using the in situ time-resolved laser extinction method coupled to ex-situ Scanning Electron Microscopy analyses. This experimental study is carried out using low pressure/high density argon dual plasma that combines microwave and pulsed direct-current (tungsten cathode) discharges. Using this plasma system, we could access either to a sputtering glow regime (microarc-free) or to a micro-arcing assisted glow regime by tuning the direct-current voltage and the pulse frequency. On the one hand, in the sputtering glow regime, only dust nanoparticles with sizes ranging from 75 to 150 nm are produced with a low density of ∼1012 m−3. On the other hand, in the micro-arc-assisted glow regime, microparticles in the range of 1–5 μm are produced along with a strong enhancement of nanoparticle production with density up to ∼1014 m−3, and a broad size distribution ranging from ∼30 nm to 700 nm is observed. We show that micro-arcing not only results in micro-meter sized particles through melting/solidification but also strongly promotes the production of nanoparticles in the 100 nm size range.

Influence of a magnetic field on the formation of carbon dust particles in very low-pressure high-density plasmas

Carbon dust particles with radius of a few hundreds of nanometers are formed in a steady-state plasma produced by an electromagnetic surface wave at 200 MHz in very low-pressure conditions. The influence of an applied magnetic field is evidenced by ex situ methods (scanning and transmission electron microscopy, Raman spectroscopy). The observed spherical dust particles are formed of aggregates of graphite nanocrystals embedded in amorphous carbon shells. In addition, the size of the dust particles increases with the magnetic field intensity. Growth occurs as the particles are confined near the sheath edge and their confinement is improved by applying a magnetic field as the corresponding electric field increases in this region. As a consequence, the average dust particle radius is directly related to the magnetic field intensity.

Low Temperature Growth of Carbon Nanostructures by Radio Frequency‐Plasma Enhanced Chemical Vapor Deposition (Low Temperature Growth of Carbon Nanostructures by RF‐PECVD)

This paper reports the growth of carbon nanostructures on Si (100) substrate in the absence of catalyst using radio‐frequency plasma enhanced chemical vapor deposition (RF‐PECVD). A variety of carbon nanostructures have been grown by low pressure high density plasma process at 400°C using a methane/argon mixture. Various shapes and structures including novel carbon nanoparticles have been found. The surface morphology was studied by Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) while the chemical composition was studied using Energy Dispersive Spectroscopy (EDS). The AFM images show that the nanostructures are predominantly either particulate (spherical) in nature or have a sheet‐like morphology. SEM images are in good correspondence to the AFM results and indicate formation of either individual islands or two‐dimensional nanosheet‐like structures. The crux of this work is in the synthesis of the carbon nanostructures at comparatively low fabrication temperature (400°C) as compared to other techniques.

低圧力・高密度プラズマの新しい展開:EOR, ヘリコン波および誘導結合型プラズマ

低圧力・高密度プラズマの新しい展開:EOR, ヘリコン波および誘導結合型プラズマ

Profile evolution simulator for sputtering and ion-enhanced chemical etching

A plasma etching profile simulator was developed to investigate the evolution of pattern profiles in various materials under different plasma conditions. This simulator is based on a two-dimensional cellular method. The model is fed with input parameters that include angular dependent etch yield, ion and neutral angular distribution, and plasma and material characteristics. It has been tested by comparison with published profiles of Si sputtering and SiO2 ion-assisted chemical etching in argon and chlorine plasmas. Observed microtrenching and bowing have been well reproduced by the simulator. The simulator was further used to examine etching for dimensions below nanometer in low-pressure high-density plasmas. In the case of Si sputtering, trenches of 100 nm depth and 30 nm or less width show unusual lateral etching. Finally, the effect of positive charge accumulation on an insulated mask resulting from negative bias voltage on the wafer was studied. This charge accumulation causes a deflection of ion trajectories. Considering this phenomenon, very isotropic etched profiles were found, in good agreement with in-house experimental profiles of platinum sputtering in argon plasma. The simulator developed is intended to be used for any material and mask combination in order to predict the profile evolution under various plasma conditions and pattern dimensions from micrometer to nanometer.

Deposition of dielectrics using a matrix distributed electron cyclotron resonance plasma enhanced chemical vapor deposition system

We have studied the deposition of silicon oxide, oxynitrides and silicon nitride in a novel, scalable, low-pressure high-density plasma system based on the ECR effect and the matrix arrangement of microwave antennas. Silane, nitrogen and oxygen gases were used as precursors. Films were deposited onto different heated and unheated substrates, including silicon, glass and polymer. The influence of the magnetic field configuration, gas flows, reactor pressure, microwave power and radio frequency bias on the uniformity and properties of the films was studied. Ex-situ and in-situ UV-visible spectroscopic phase-modulated ellipsometry and FTIR spectroscopy were used to study the properties of materials and the uniformity of the depositions, along with electrical measurements to evaluate the dielectric properties of the films. Optimizing the magnetic field configuration and applying an RF bias, we were able to achieve an uniformity better than 3.3% across the 200 mm wafers. Stoichiometric silicon oxide films with index values matching that of thermal silica were grown on unheated substrates with rates exceeding 2 nm/s.

Low temperature plasma carbon nanotubes growth on patterned catalyst

This paper reports on carbon nanotubes (CNT) arrays grown on patterned catalysts by plasma-enhanced chemical vapor deposition (PECVD). CNT growth has been performed by low pressure high-density plasma process at 700 °C in acetylene/ammonia mixture. Prior to CNT growth, a standard lift-off process has been used for catalyst patterning. The chemical impact of the lift-off sequence on the catalyst activity is discussed with respect to the amount of Ni pre-deposited. In particular, it discusses the impact of this wet treatment on CNT growth when the catalyst amount is lower than a threshold value.

Characterization of carbon nanotubes and carbon nitride nanofibres synthesized by PECVD

This paper presents a novel dual plasma enhanced chemical vapor deposition (PECVD) process developed to grow carbon nitride nanofibres (CN-NFs) at room temperature (RT) and its basic version used to grow multi-walled carbon nanotubes (MWCNTs) at temperatures as low as 550 °C. The dual process alternates two low pressure-high density plasmas, one inductively coupled (ICP) and the other one excited by distributed electron cyclotron resonance (DECR). MWCNTs can be synthesized using only the DECR plasma source. The paper focuses on the comparison between CN-NFs and MWCNTs detailing their structure as revealed by transmission electron microscopy (TEM), X-ray photo electron spectroscopy (XPS) and Raman spectroscopy.

PIC-MCC Simulation for Plasma Immersion Ion Implantation Processing

Plasma Immersion Ion Implantation (PIII) has been developed as a method for high-flux implantation and conformal implantation on a complex shaped target. In PIII, a negative pulsed high voltage is applied to the target immersed in low-pressure high-density plasma. Then an ion sheath is formed around the target and energetic ions are implanted on the target surface. By increasing the plasma density, conformal implantation is possible. However, this process can not be easily realized for a complex shaped target, for instance which has a trench or holes with high aspectratios. In order to find the best condition in the process, it is very important to know the sheath shape around the target and the energy and flux distributions of implanted ions at each surface point. Plasma behavior in the PIII process has been simulated using “PEGASUS”.

Profile control of high aspect ratio trenches of silicon. II. Study of the mechanisms responsible for local bowing formation and elimination of this effect

Profile control of deep and narrow anisotropic trenches in silicon has been investigated in a low-pressure high-density plasma reactor working with a SF6/O2 plasma mixture. The aim is to achieve deep trenches with high aspect ratio (depth/width>10), high anisotropy, high selectivity (silicon/SiO2 mask>300:1), high etch rate (>5 μm/min) and with no local bowing. This study focuses on a cryogenic etching method. In the previous article [M. Boufnichel et al., J. Vac. Sci. Technol. B 20, 1508 (2002)] we presented a parametrical study of the evolution of local bowing as a function of the process conditions. Here we present a complete study of the mechanisms involved in the creation of local bowing at the top of deep narrow trenches, and in particular the effect (influence) of the characteristics (nature, thickness, and side slope) on local bowing. Explanations for the appearance of local bowing are given and targeted experiments are performed to eliminate it. The final performance shows that we are now able to etch a minimum of 60 μm in depth for a 4 μm trench aperture with no local bowing at a high etch rate of about 5.5 μm/min with a cryogenic method in which no cleaning step is necessary unlike for the Bosh process [M. J. Walker, Proc. SPIE 4407 (2001)].

Development of an RF Plasma Source with Mirror Field for a Compact Neutron Generator

An RF plasma source has been designed and constructed for a compact neutron generator. The generation of neutrons is based on the D-D/D-T fusion reactions, producing 2.5/14.1MeV neutrons, respectively. The neutron yield of this device depends on the density of D or T monoatomic beams, which can be extracted from low-pressure high-density plasma sources, especially RF-driven plasma sources, such as an ICP (inductively coupled plasma) or a Helicon plasma source. In this presentation, the design of an ICP source with mirror field has been performed arranging targets in coaxial geometry. Plasma targets without any solid target as well as solid targets are arranged in this design for the comparison study and the effects of the mirror field on the plasma properties will be evaluated.

Dynamic model and electrical characteristics for rf-biased electronegative plasma sheath

The dynamic model for the sheath of an RF-biased electronegative plasma is developed. The low-pressure high-density plasma conditions are assumed and considered as an electric-field-free boundary. The Bohm criterion for collisionless sheath is assumed to be valid and the presheath region is separately considered in calculating the sheath properties. The electron density n/sub e/ and the electronegativity /spl alpha/ /spl equiv/ n/sub -//n/sub e/ are taken to be determined independently of the sheath dynamics. The time-resolved wave forms of the sheath potential and thickness are obtained. The characteristics for the RF power dissipation mechanisms are also investigated. The RF bias power is assumed to be dissipated through two power loss channels - the first is due to ion acceleration in the sheath and the second is due to the electron heating at the plasma-sheath boundary. From the power loss point of view, the equivalent circuit model is developed and the sheath impedance is calculated. The electronegativity, as well as the current and RF bias frequency, is found to be an important factor in determining the electrical characteristics for the RF-biased electronegative plasma sheath.

Loss kinetics of carbon atoms in low-pressure high density plasmas

Vacuum ultraviolet absorption spectroscopy (VUVAS) with a carbon hollow cathode lamp was applied to the measurement of decay rate of C atom density in the afterglow of CO and CO/H2 inductively coupled plasmas. The transition line used for the measurement was 2p3s 3P2–2p2 3P2 at 165.7 nm. The influence of background absorption by the species in plasma other than C atoms on the transition line of C atoms was found to be negligible. This was clarified by measuring the absorption intensities around the center wavelength of C atoms in plasmas with VUVAS employing a xenon microhollow cathode lamp. Moreover, the dependence of the decay rate of C atom density on pressure revealed that C atoms were dominantly lost at the surface rather than in the gas phase in both CO and CO/H2 plasmas. However, in the case of CO/H2 plasma at higher pressures over about 5.0 Pa, C atoms were lost in the gas phase as well as at the surface. The diffusion constants of C atoms in both CO and CO/H2 plasmas were also determined to be 3.1×104 and 3.7×104 cm2 Pa s−1, respectively.

Si etching rate calculation for low pressure high density plasma source using Cl2 gas

Based on a simplified Cl2 plasma Si etching mechanism, we calculate the Si etching rate with a comprehensive analysis of the effect of ion bombardment. With its flux distribution, the bombarding ions are regarded as the sum of independent monoenergetic beams (SIMB) approach. The Si etching rate is examined over the radio-frequency (rf) frequency (νrf) range from 1 to 20 MHz (0.1νpi≲νrf≲2νpi, where νpi is the ion plasma frequency) under the typical low pressure high density plasma condition. To consolidate the SIMB approach, the etching rate is compared with the result from the monoenergetic single ion beam approach. The difference in the Si etching rate between the two approaches is notable at a low rf frequency range. The effect of threshold energy on the Si etching mechanism is also investigated. We conclude that under a low rf frequency, for a precise etching property examination, the effect of the bombarding ions should be investigated through the SIMB approach.

Cryogenic etching of deep narrow trenches in silicon

Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 μm wide trenches etched to a depth of 50 μm at an average etch rate of 5 μm/min with highly anisotropic profiles and very high selectivity (>500:1) toward the SiO2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10 °C between the chuck and the wafer is expected for thin wafers (210 μm).

Self-consistent kinetic simulation of plasmas

An improved version of a kinetic simulation technique, the convected scheme (CS), is described. We present a ``long-lived moving cell'' version of the convected scheme and describe a recipe for implementation in detail. In collisional systems, the numerical diffusion in ``ballistic motion'' does not affect the simulation result of the regular CS, which can be set up to minimize numerical diffusion in collision processes. With the new scheme, the numerical diffusion is highly reduced even in nearly collisionless plasma systems, which is important in the modeling of low-pressure high-density plasma sources.

Temperature dependence of inductively coupled plasma assisted growth of TiN thin films

Abstract The use of low pressure high density plasmas to assist the synthesis of ceramic thin film materials is in its infancy. Using an inductively coupled plasma assisted magnetron sputtering system, we examine the dependence of plasma-assisted growth of TiN thin films on growth temperature at different ratios of ion flux to neutral atom flux. Our results indicate that a temperature independent densification of TiN films occurs above a certain ion to neutral atom flux ratio. As an example of this temperature independent densification, we demonstrate the formation of dense B1 TiN crystalline thin films at growth temperatures down to ∼100 K.